I have the most subborn daughter, she thinks she is the princess and runs the house. I tried potty training her right after she turned 2 and failed. I put the potty away for about 3 months and tried again. This time I had a doll that peed in the potty, when the doll peed we had a huge party with noise makers and dancing. Well she saw all the attention that the doll was getting and wanted it for herself. She will be 3 in February and is totally potty trained.
On another note...maybe you should wait a little while until he gets used to having anew sibling. He sees the baby getting changed and most of the attention from mom. My sister and I are very close and she has a 2 month old baby. My daughter sees him getting the attention including diaper changing and she has asked on several occasions for me to change her!
Good luck!!!
PS I am a mom of 3 - Devin 5, Donovan 4, Myalyn 2 (going on 16!)
Sam