He was an underage kid (18 or 19 because he was in college) who had one beer and has a super high alcohol tolerance. It even mentions at the end that he was never drunk that time. He understands the first night he spends the night with Zama that it could and probably will lead to sex if he stays again. That's why his inner monologue when he's in the shower on the first night says something about "not realizing he was that type of person". He just uses the alcohol as an excuse to have sex without putting too much thought into it at the time. It's the kid's own personal justification. None of that equals rapey to me.
A lot of people need to justify their need to just go out and "get some". I personally believe it's because of the stigma that surrounds sex as a bodily need and not necessarily for love. Not that love or monogamy are bad in any way, but it's not always for everyone or relevant to every situation. It definitely did come across as very casual and spontaneous within the manga, though, and it did all work out in the long run. :)
He did act a lot younger than his age. For me, I think it was the super strong attachment that he had for his "father" that gave this impression. I never really got any feeling of sexual attraction towards his father despite the uke calling for him during sex, though. I think it was a combination of him being depressed over his father not having the same attachment to the place they met and, I got the impression, that the uke had never had someone to care about at all until he met his father. These things gave off the feeling of a much younger emotional maturity level than would usually be seen in someone his age.
The first time they had sex felt way too rapey for me. He is an underage kid who was just drinking alcohol for god's sake. The rest was pretty boring and hard to follow. I usually love this mangaka's work, so I was sad.