Hi, N.. Well, there could be a number of reasons. The first that comes to mind is that maybe it's too cold in the room for him. If he's 8 months old here in December, then he was born in April and has been used to the Spring/Summer weather of the first few months of his life. Now the air is colder. Maybe he just doesn't want his little bottom to hit the cold air.
Does he have diaper rash? When air hits diaper rash, OUCH! Babies will cry for that reason, too, and not want you to touch them for diaper changes.
He could also be an active baby and not want to stop what he is doing for diaper changes, which are boring to him. Try putting some kind of distraction above him while you change him. I used to change my son on a padded dresser, and I would hang toys on strings above his head so he could play with them while I changed him. I also changed him on his bed sometimes where I had put a similar mobile type thing.
Make sure your hands are warm every time you change him. Running your hands under warm water before touching him will do the trick nicely. Also, make sure the room is warm, and make sure there is no cold draft like the air conditioner vent or a fan blowing on him. My son hated to be touched with cold hands, which was a problem while we were living in New York!
What surface are you changing him on? It is possible that he has a boo-boo or a sunburn on his back, and if you are putting him on a fairly hard surface, that could be hurting him.
I hope you figure out what the problem is, and that things return to peaceful diaper changes.
Peace,
Syl