Its not distributing alcohol to minors if you are their guardian and its on private property. I had a friend who had a german parent and had beer with dinner since he was like 12.There's a lot of adults in these shoujo mangas that could probably be charged with distributing alcohol to minors. It's definitely on the weirder side of tropes...
Yeah it's one of the few disadvantages of exercise and clean living. I remember getting hung over an hour after drinking back when I was more serious about that.Aside from "because funny" IIRC athletic types tend to have lower alcohol tolerance due to having a higher metabolism, along with something to do with weight.
So he's a relagively lanky former soccer player with zero experience with alcohol. Add in the possibility that he's a lightweight as well, and... maybe?