My first two daughters were 4 by the time they were potty trained- there's nothing that says they have to be potty trained at 2. There may be some deeper emotional issues he's dealing with, as it is totally normal for a child to regress when a new baby comes into the family. My 20 month old did- she potty trained herself and then when the baby was born, she stopped using the potty. She's now almost 3 and is finally using the potty- it took her nearly a year!
Give him lots of cuddles and praise and some 'big boy' tasks, especially some that help him feel a part of caring for the baby. Bribe him to SIT on the potty, and if he actually potties in it make a big deal out of it. Go all the way back to square one. You may even need a big prize- if you use the potty all week without pottying in your diaper or underwear or floor then you get (a spiderman cake, a game, ect.)
For my almost 3 and 4 year olds I did this with the cake but I also have had to let them go bare bottomed. panties don't work- they pee in them. But if they are naked they are much better are going to the potty. I also have to remind them everyonce in a while but they are getting pretty good at going to the potty on their own.
S., mom to four girls ages 1-5