B.S.
First, mom, you don't ASK him anything! You tell him firmly that it's time to go potty and you take him and sit him there. You will probably have to sit with him for the time being. You can chat with him, you can sing songs with him, or read to him. If you do this on a regular basis, you will eventually *catch* him and then you'll need to give him lots of praise....even a treat. I give my daycare children a sticker for their *potty report* that hangs on my door....one they can take home each day. Some parents give a piece of candy. You can use fruit. Just whatever you have on hand that you can give him each time he succeeds....but *only* if he succeeds. He's old enough that he should understand and it should take just a few days, if you stay consistent and focus on it until it's done. You don't punish him for an accident at this time, nor do you scold him. You explain that it goes in the potty.
Now, I realize you'd like him potty trained before the baby comes, and it's possible; but, some siblings regress for a short time when a new baby comes. Not always.
There will be lots of accidents over the next year or so; but the older a child is when you potty train, the easier it is and the better they remember.
The daycare/mdo staff take the children to potty on a regular basis as part of their routine. It will help them, if you can get him started at home, doing it the same way, once he shows he understands.
Good luck and God Bless..