Try leaving him for say about 20 minutes, then go in then leave him for 30minutes and go in etc. I found it worked with my kids when they were small - the dogs are kids too LOL.

Again, I was really lucky with Maxxy, re; toilet training - his first Mummy was really hands on with the pups and took them outside regularly so he was already half way there by the time we got him at 15 weeks