OMG. What a stupid thing to discuss together. Next time, ask him about the vaccinations in Africa, it's such an hot topic
Now, I suggest you to invite you in your house to watch together something, possibly a sport event, as you have to project the imagine of an athlete.
So, the idea is to watch a sport event together (something that you like first, if there is nothing a TV show or a series would be OK). Your house will be perfectly cleaned and tidied up, he is studying medicine so he has a certain idea of himself five years from now, and it does not include living in an house with a pair of used jockstrap on view
Also, you will provide some healthy food, no comfort food, no junk food, but also some beer or wine.
You will be dressed nicely, could be a bit on the informal side, but not as if you're going to the gym.
After the TV show, you'll start your show. Remember that at the end of this, either you've fucked him, you are only friends or he decides to cut you off. So, play accordingly.
You'll say something about his body and his diet, like you like his torso, he has very good abs, that you like something of him and you want to have the same. Don't be a sissy saying that you don't like something of you. Then you say that you'd like to see those specific muscles of him, there is a good light and it's better than in the gym. Make a connection with what happens in the gym, so if he is embarrassed tell him that you've been naked many times together, you are between men and there is nothing to be ashamed of. The goal is to have him shirtless, and you should be shirtless too, maybe saying something like his abs are better than yours, or your legs are better than his, the more you are undressed the better.
At this point, if he is gay he will got the bait, but it's possible that he is embarrassed, confused or that he is a straight man, so after some reciprocal checking out, you should say something like 'OK, my whatever is bigger than your whatever, but your dick is bigger'. If he is surprised, tell him there is nothing to be ashamed of, you've been often in the shower together, why don't you check this out? If he is doubtful, insist on the men being men thing, and once he takes his dick out, as soon as one of you starts having an hard on, say that this is a better way to compare, go on your knees and start sucking him, as he could be a straight curious man and you have to provide a comfortable setting for him.