Jake and his humping habit

Jake started to hump my socks at around 13 weeks...Here is my old thread about this:
http://www.thecatsite.com/forums/sho...ight=jake+sock

He is now neutered and much bigger than he was then, he is well over 10 pounds and is 7 months old. So, he has moved on from socks to jackets and shirts I'm assuming due to him growing. It's seriously scaring me because when I was away for 2 weeks he lived with my boyfriend and he never did this then but then when I got him he did the very same day I don't get it!! He humps something every single day now. It's pretty much the same as when he was a kitten, purring very loudly etc. Now he will pick up a jacket off of my bed and while holding it in his mouth and purring he will walk around my bed and then hump it for a bit, then carry it down and around the room doing the same and sometimes carries it out of the room. I'm just worried why would he be doing this? He only does it picking things up from a specific place, he will never go up to the couch and hump a jacket standing there, it's only if it is standing on my bed. He was neutered at 3 and a half months and I thought he would outgrow it or stop after being neutered but nope Does anyone know any reason why he might do this?