The part where you say that sometimes you go to touch your boyfriend's penis "to initiate arousal" -- maybe he wants to get an erection with his own timing as erotic events unfold between you, instead of having you cause him to get one.
Eh, it's just his preference. I think this is a good opportunity for an interesting conversation. Ask him what? When it started? What prompts his continuing to do it? See how he answers. I mean, maybe his mom made him do it when he was little. And it's a habit now. So, chat him up about it. Could be a good time to get to know each other even better!
So headline point is I would guess 2 likely reasons....either he is trying to make his penis longer or he is toying with the idea of being trans/gay or at least ha an interest in it.
So now to the detail of why I can guess those reasons.......basically from what I know and have seen of tucking, it pulls back the penis and stretches it out longer than normal and keeps it in this state (imagine a elastic band and then having that band stretched constantly). Now there is a good theory that constant tucking would mean the penis gets used to being stretched and overtime accommodates and grows longer.....where does that theory come from and how is it evidenced?...Transsexual or shemale men that do everything to speak and look like a woman except surgery to remove the penis.
Many people who comment on trans pornography often say things like "wow big penis" and "why do most trannys seem to have bigger penis than straight guys" and also things like "why do alot of trannys have curved/bent penis"....the answer to both those common observations and comments can be many reasons but one of them is certainly the constant tucking which being a trans man would bring.....they will always be tucking while wearing clothes outside and of course as they sit in public. You can imagine the stress the penis would go under eachday and the funny directions it would go into as you move or sit etc......it's easy to see how it can develop in length and also get a bend.
Tranny porn is aimed at people being attracted by the fake female bits like face and breasts and bottom but also with an alluring penis, so most tranny pornstars need to have a big penis. (btw a trick to notice if a penis has been lengthed is the colour, as more blood stress will make it darker or patchy and also if it has long or prominent visible veins which is a bit like when you see those bodybuilders with veins everywhere)
So after reading your question and the other answers here, I would say you are right to wonder why and perhaps ask more details which can be backed up.....yes you shouldn't jump to conclusions and this may open a can of worms as the saying goes, but you have the right to know and make your mind up about if you want to accept someone like this/help them - whatever the case may be.
The answer that he used to do this to stop others seeing an erection is a bit ominous.....yes it sort of sounds like a possibility as a teen but unlikely, and he's not a teen anymore is he and nobody would tuck while sitting out of habit. All trans men would rather not have that bother of tucking or uncomfort that comes from it.
Now is this all crazy words if he is indeed a straight man? No not crazy as many men who are straight will still view and enjoy tranny porn.
Is it crazy words if he indeed has a normal or even bigger than normal penis? No not crazy, because men addicted to porn can easily get hooked on being bigger and it doesn't matter if they know they have a good size penis or if girls are satisfied with it, they want to be bigger for themselves. They also are unlikely to like to admit to someone that they are trying to make their penis bigger.
I'm answering this as a straight alpha man who has sadly seen alot of pornography, and spoken/shared with others about this. I've also played football (soccer) most of my life and been around naked men in changing rooms, and men of the pornography mentality.
In short it is definitely not normal to be tucking, and again I'd think about whether you want to either accept living with a person like this, help this person improve themselves, or not live with them at all.
Everything I've suggested may turn out to be incorrect or not fully correct in your situation but at least some of it is likely and will be true for others in a similar situation.