Why is my TB starting to be nasty?

Im 17 and i have had my TB for 10months, hes only 9 and resently has been seeing the vet for his teath he is ok naw. Im trying to put a lot of weight on him so i can ride him agen as he is so under weight, and also make him happy. I lunge him almost every night and have just stared puting side rains on. but he is being so nasty and biting every one and all the other horses no matter what you do he puts his nasty face on. is it because hes so skinny????? if no what can i do to help him????? Rachel