Disclaimer: the next follow my playthrough. It is Bella's path. Seems like my mind can only comprehend following Bella's routes, no matter how hard I try. So I don't know about others routes.
Zero: sure, disregard all my other comments that pretty much answered your point.
First, where did you see a horny teenager? Who did he try to pick? The most, he tried to be supportive to Vic and (not like you can blame him, given how awesome she is, probably a bit too) friendly to Nami. But I have not seen him flirt with anyone. He is still the same impotent he was. In my playthrough, very confused because he is "falling for a horrible person" (his words).
Second, again, people have highs and lows. Unless you understand this, don't try to "rationalize" how depressive people function. We are not all the time hidden in our bedroom listening eerie music with the intent of cutting our wrists.
Third, he is trying to do get out of his depressive side. He went to have a drink with everyone, he went to help Vic when she Maja called and promised to help her. He is going to therapy, he has Nami and Nojiko helping him. Cut the man some slack, of enjoying a sport he liked until Summer's disappearance!