my horse kips nipping me when i groom him

I have had biscuit now for nearly 3months. He is 5yrs and an ottb.
He has been an absolute joy to us and settled into his new home really well. But recently he has started nipping me when I groom him, especially around the back and flank area.
I talk to him softly when I groom these area's but im just wondering if he is playing with me or is this behaviour turning into something more serious?
Has anyone else had this problem with their horses?
Other women @ the stables say he is probably just playing and to give him a quick slap, but im not so sure. I don't want to start hiting him for no reason.
Has anyone any suggestions?

First of all, we need to address whether this nipping is just a behavioral issue or whether it occurs from pain.

Does he only nip when you brush a certain area? Try running just your bare hands over it. Does he still nip? Now gently and I mean very gently press. He may just not like being touched there and you'll have to discourage the nipping. But if he only nips when pressure is applied, he may have hurt himself. Its hard to say over the internet, but I'd recommend maybe getting an equine chiropractor out to have a look at him.