Little sore on my cat's neck?

have a 3 year old Ragdoll cat,Riley, that I just noticed last night has a sore on his neck. I thought it may have been from my other Persian cat when they were playing but I am not so sure. I use Frontline every month so i don't think it is fleas. Riley had a urininary tract infection so I feed him science diet C/D. I also use feline pine cat litter. I haven't changed his food or litter brand recently. I thought it was a bite from my other cat but what else could it be. The hair is missing from around the wound and it is oozing just a little little bit. Riley is eating and playing as usual.