Litterbox problems

My grandchildren found a female cat with a kitten in the woods by their home. The mother most likely had given birth in the woods and that was the only kitten that they had found. My daughter and her family adopted the kitten. He is now 7 months old and has been neutered. Joey is a happy kitten and joined three adult female cats in the home;they seem to get along pretty well. The only problem is that Joey has been peeing in my granddaughters bed; three times in the last couple of days. He does use the litterbox,too. There are 3 litterboxes in their home for the cats. Joey is going to the vet to find out if he could possibly have a uti. But if not, what would cause him to do that? What could be done to help him stop doing that?

 

I think its the litter box that is causing the problem