Saying that a horse is favoring his leg could mean many different things. It could be that his back is bothering him, his shoulder/hip could be sore, could be his knee, his cannon bone, his hoof, his coronet band. It is probably best if you had somone that might know come out and take and look at him and then call the vet if they think that the problem could be worse.
As FehrGroundRanch said, it could be anything causing that. What I can guarantee you it isn't is good. It could be originating from his back, shoulder/hip, knee/hock, coffin bones, hooves, or anything in between.
If it's not too too bad, walk him around a bit and see if he if he'll walk out of it... if not, or if it persists, definitely call your vet. Or if you have a trainer, have him/her look at your horse.
Hope everything's ok:)