He is probably swollen and a bit sore and it wouldn't be unusual for him to try as long as he can to not pee till he just can't hold on any longer. He may have still been a bit under the influence of his anaesthetic and peed in his sleep. However this should have been explained to you with your discharge notes.
I agree you should call your Vet and enquire. For one thing, you sure don't want infection to set in because urine got into his suture areas. Your Vet may give you tips on how to gently clean the area.