Shitty father?? & potty training

I divorced my ex husband 2 years ago we aren’t co-parenting well. He takes our kids every weekends our son is 4 years old and Is having a hard time potty training I decided to dedicate 1 whole week of it I took time off work etc well I got my kids back yesterday & guess what? My son had a diaper on I immediately took it off him & he peed all over my bed & himself I was so upset & called my ex he said that I should NOT be forcing him & that he’s too little to learn