K.M.
When I was little and completely potty trained once my sister was born I regressed also I even went to get the diaper and tell them I needed to be changed. I guess it is a little scary for kids to think there will be someone to compete with. I am sure you have talked to your son about the new baby. When I was expecting my second child I told my daughter that we would have so much fun giving the baby a bath and feeding him and playing with him and that I would need her help. From the minute he was born she helped me and I included her in everything I did for him. She even woke up around the clock and would come out and sit and watch me feed him I can still see her sitting there. She was 3 1/2 years old exactly to the day when he was born. She was his Mom too LOL. Good Luck just be really excited when you talk about the new baby and how much you will need his help. Maybe even say the new baby will need the diapers so you will need to help and be a big boy and go on the potty so the new baby can have the diaper. I hope this helps and congratulations on your new soon to be baby.
Kay