Perhaps it is his way of coping with his own feelings. He tends to do the most ridiculous things as an outlet. Both of you are still young yet. Doing such things, as nonsensical and pointless as they seem, is all part of the growing stage.

[ yeah, that's right. your dick drawing is GROWTH, man. take that. ]

Or... Maybe forcing him to partake in your own brand of shenanigans is a way for you to have an outlet?