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.

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.

Depending on his age and on his stage of sexual maturity, he may be unexperienced with the whole erection thing. This is not the norm, but it might be your case. Even if he is experienced - aka not unfamiliar with erections and even masturbation - he may feel a bit confused, his thoughts 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). He might feel it as inappropriate, happening in the worst moment and a bother.

In the end it depends on how you want to play it; since he remembers it after so much time, I'm assuming it was quite a relevant event.

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.