D.R.
If ya' can't beat 'em, join 'em. Maybe he won't poop before his bath, but have the potty seat on the toilet and ready to go. Just be aware of any cues that he's going to squeeze one out, and then pull him out of the bath and onto the potty. A couple of weeks ago my then 12 month old started making the face and the whine while in the tub, and I quickly found the potty seat under the sink and plopped him on the toilet. He did his business, though he wasn't horribly thrilled about getting out of the tub. At the very least, it will put the connection into your son's head about the feeling of having to poop and going on the potty. Good luck!