that one guy...

I got all the seasons to this show as gifts and have watched all the episodes in order, and there is an obvious change in hair. The most obvious is the leap from season 1 to season 2, but I think a lot of the difference really can be contributed to hair style. The slicked back look makes it look much much thinner. In season 2 the thinning is less noticeable because he stopped gelling it down but you can still see it if you look. He still has obvious recession if you look at where his hair is parted. It's very thing through there. As you look at each episode in order, you can see this area very slowly thicken out episode by episode. From season 2 to the current episodes, he has the same hairstyle but it does not appear to be thinning at all now along the part, and is thicker overall. There was one mention of him using propecia, in season 3 or 4. I can't remember the exact line, it had something to do with him telling pregnant janice to stay away from his propecia, something like that.