A.N.
Since the baby came and then this regression started? You can't discount it.
Try letting him know how glad you are he is a big boy now and even how nasty and stinky the baby is for doing it - really!
The baby won't know or care!
He will see you play and coo to the baby when you change it so make sure he gets this bonding with you also at certain special times? Explain why it's different for him.
You don't want him to join in for the attention it gets him! It's natural that he doesn't want to bother, you persevere with the treats etc and every time he uses the potty praise him lots and when things don't work out ... change him in silence.