How do I get my 5 year old son to stop pooping in bathtub after being so consistent with the potty for 2 years?

I have tried positive reinforcement, stickers, being extra strict, and smaller bathtubs. My child is 5 years old and was potty trained at 3 and has not had an accident in 2 years. This is becoming a worrying problem. What else can I try to make him stop?