We don’t have this problem but my niece didn’t poo in her nappies from very very young so my sister just took the jump and kept holding her above the toilet to poo. She was daytime potty trained by 16m!! As above, elimination communication can work! It’ll be a lot of work but worth a try and is also easier than cleaning the bath every day 😂
I'd also recommend that! We are doing elimination communication and we catch about 75% of his poops in a potty
Could it potentially be he’s a little backed up and the hot water around his belly is what’s making him go?
This is a bit out there but try him on a little potty/ the sink before you put him in the bath. Sounds silly but look up elimination communication