In my novel, I'm currently writing from a male's perspective and I need a bit of help. It's not an erotic book, but I'm writing a flashback from his teenage years which involves a bosom hug and I think it's realistic and fitting to mention something subtle about him getting excited but I have absolutely no idea how to phrase it.
My character is a very reserved individual, who would never use your typical terminology. In a previous chapter, he referred to masturbation cryptically as a "catharsis" (which was fitting in the context).
You could be vague and mention him feeling his trousers or pants "getting tighter", or something of the like. It's pretty much the standard imagery used to represent an erection without actually mentioning anything, and it's also a quite accurate physical description that your male readers will have no problems identifying.
For pretty much the same reasons, you can mention him having troubles walking due to his trousers becoming slightly uncomfortable.
This is pretty much everything you can say (that's coming to my mind) about the physical feelings without going into details.
Psychologically, you can mention him being embarassed in his excitement: he could be afraid of the other character becoming aware of his erection during the hug. Again, this would be realistic and you can easily say something along the lines of "[he] feared [the other character] noticing his arousal" and you'd be ok.
Since he's in his teenager years, he may be unexperienced with the whole erection thing - he may feel a bit confused, his toughts will lose a bit of focus, and he may shift his attention on his bodily sensations (the softness of the hug against his body, the scent of the other character).
Arousal may make him want more of that pleasurable contact, like reciprocating the hug, or again he may stare at the other character with a different gaze.
So, there's quite a lot you could say depending on how the character feels about the erection (excitement vs lust vs embarassment). Let me know if it helped.
Liquid's answer is good, but I would like to specifically focus on one way of doing it; chiefly, mention the awkward feelings one gets in such situations. Men, especially young men, aren't exactly comfortable with inappropriately-timed erections, so given this is from the male's perspective, you have a lot of fun ways to play this off. For example:
Myrtle squeezed her soft chest against me. I'd be lying if I said it didn't feel good. Then and there, the only thing on my mind was how to make this last forever. Of course, that very thought made my body sabotage the entire effort. It was touching her leg, there was no way she wouldn't notice. Within moments, Myrtle let me go with a polite smile; she said nothing, but we both knew the reason.
This question borders on "what to write" instead of "how to write", so I will concentrate on the latter.
If your character has masturbated, he has had erections. Probably every day, since teen age boys do. If he is "reserved" and doesn't like to think in crude terms, then he will have invented his own words for this phenomenon.
Changing "masturbation" to "catharsis" is focusing on the emotional result instead of the activity. I would focus on a similar language transition for an erection: Focus more on the emotional result.
I think "tight trousers" is a cliché, and doesn't really convey what the character is feeling. It is also not entirely accurate, men and boys can wear brief and keep their penis angled down, in that case they can have an erection that doesn't show. Most boys figure that out early, and don't "pitch a tent", as the slang goes.
Also, erections are not all or nothing; they can vary from "chub" to full staff, depending on the level of arousal. For somebody in a bosom hug, I wouldn't assume a full and raging erection; I'd guess the boy had the beginning of one, a partial erection.
Were I writing, I'd focus on the emotions this causes. Between males and females, I'd guess the frequency of unwanted or inappropriate sexual arousal is higher in males, and the sensations may differ somewhat (although engorgement and increased sensitivity are common to both genders), but I'd also guess the emotions of unwanted or inappropriate sexual arousal would be similar, and by focusing on those we can cross the gender-gap.
I also wouldn't try to summarize this in a word or two; if this is an infrequent occurrence in the story, you don't need a shorthand. Allow yourself a sentence or two, to convey the feeling accurately. Remember, readers don't mind reading.
Don't mention the erection at all. As he is about to be smothered in breasts, switch to descriptions of everything he is using to mentally distract himself from having a sexual reaction.
Perhaps, make it more of a "battle", attacking one of his senses at a time. First the image of her breasts up close which he fights back with other visual imagery that is very un-breastlike. Then as he seems to have that won, his next battle is her smell which is layers of feminine things like soap and powders. He battles this back with memories of foul smells. Last it is the feeling of contact, skin to skin, and he has to knock it away with very un-sexual human contact. (Bonus points if the replacement objects resonate thematically or have character-arc significance.)
After this epic battle of his senses – which of course was just a few seconds but in his mind lasted an eternity, the reader can get a hint of how well it worked the same way he will know. He "adjusts" how he is sitting.
The possibilities are endless:
"Chubby Checker is doing his stand up routine"
"From little acorns, mighty oaks do grow."
"You know The Lark Ascending by Vaughan Williams? Well, the lark has ascended."
Pretty much anything in that situation that a young man says, which refers to anything rising or growing, is going to be construed as being a reference to an erection.
If a teenage boy feels sexually excited, that's all you need to say and male readers will understand.
A roundabout way to describe is having to rearrange his equipment or having to hide his excitement by holding a book over his lap.
Teenage boys use words like "erection" and "hard on" all the time.