Autism and potty training

My son is 4 years old and has level 3 autism. He goes to aba therapy 5 days a week for 5.5hours a day. The entire day that he is there he wears underwear and will go potty every single time. He even initiates it which is something we struggled with in the beginning. He hasn't had an accident there in two months.

But, as soon as we get home it's completely different. He refuses to use the bathroom. He screams "no way" at me if I try to initiate it. He won't initiate at all and will just pee through several pairs of pants and underwear.

Has anyone else experienced this? What did you do? We have visuals everywhere, we've tried rewards, nothing seems to work. Any advice would be extremely helpful.