I waited until bubs was born and didn't push toilet training until 34months, within 2 weeks we had no wee accidents and the poos we are still working on, but he seems to wait for a nappy for them unless he is too busy playing.

If DS is fussing over nappy changes I'd say start as my DS gets fussy now with nappy changes as we still put him in pullups when we are out to save the accidents from happening and we haven't pushed the night time training yet. My DS hasn't regressed because the baby is around.

If your little guy accepts the training all well and good if not then you have 2 in nappies. I 'd say if he is ready he'll be happy to go with it, if he isn't it won't work.