My puppy is 10 months old and was fixed at 6 months. He started to pee in his sleep and when he is laying down yesterday. What will cause this to happen. He acts normal and eats and drinks good.

Could be UTI (urinary tract infection). The vet would want a urine sample.

To be sure I would take him into the vet. Does he need to go out a lot during the day?

It could be a touch of incontinence, good thing with males is as they get older it normally goes away. I had an Am-Staff with the same issue. After his neuter, he would dribble in his sleep. Was a huge pain in the butt always having to wash bedding and his pillows, I started covering his pillow with a garbage bag, and made a slip cover with some old towels. Made life much easier. We took him to the vet, and it was nothing medical, as he got older his bladder and ureter matured and the problem went away.

It could be something a little more serious, like a UTI or a baldder infection, so it is always best to have this looked at by the vet.