The medication is making him constipated and not able to pass furballs, so there is other medication he needs to take to help with this. I have tried all the hairball remedy cat food.

I use and suggest:

Butter
Mackeral in olive oil
A teaspooon of olive oil over his food

EGG YOLK lecithin capsules, half per day. Breaks down the hair. Keep in fridge once opened.