A splash of cool water on his lower tummy would help, just make sure they get to warm back up right after. Same idea as the wet wipe on tummy before a diaper change to avoid fountaining
Stand him up in the bath supported if he's old enough? And blow some cold air onto him. With mine whenever he goes I say weewees (since he was tiny). He is 11 months old now and the last 3 times at the end of his bath he's stood up and I've said weewees and he's looked down and done it lol so maybe on to something hopefully... Worth a try?!
Mine does it with the warm water on his lower tummy.
Sorry, he’s 2 months old
How old?