So far, I see nothing controversial here.
Other than the obvious fact that he's of legal age to consent, there are several mitigating factors to their age difference:
i. For starters, women in Japan live on average six years more than men - one of the largest gaps among developed countries. So being 17 years younger means he's going to be alone, if the average is followed, for much less than she would if he were older, about 11 years.
ii. Being extra tall also reduces the lifespan, so there go away a few years more of loneliness for his widowhood.
iii. There's plenty of time for children even after he goes through college, if they even come to want children, so long as he doesn't choose medicine. Pregnancy becomes really risky at 42, so they have time to have a child if he begins working at 22. There's even time to wean the first and have a second. Women in advanced countries have children later, too - after 35 in South Korea, for example.
iv. Most importantly of all, he's clearly precocious, at the tail end of the normal distribution. He was getting aroused from her at what, nine? There's clearly a chemistry there, at least on his side. Emotionally, he looks even more mature than she is, he was clearly leading the interaction in these early chapters. It's not a law set in stone that people only become mature at a certain specified age, each brain has its own process.
Yeah, it's mildly concerning that the boy is willing to tie the knot with his first. But that doesn't have anything to do with their age gap, it would be a little concerning if he'd decided to hook up with his first high school girlfriend. Statistically, I think people who experience a bachelor's life with more than one relationship tend to have more success in marriage, but again it's not a law of life. I know people who had lots of relationships and still failed to have happy marriages, and people who lived long happy lives with their first partners.
Here's to hoping they both grow as human beings during their relationship.