My husband and I potty trained 3 boys. Each was so...different. None was successful before the age of 2.5. One as late as almost 3.5. We found that each went through a rebelious stage, to show us THEY were in control. Often these stages accompanied changes in our life (taking them out of a crib, a new brother on the way, etc.) Good news was, those stages passed, and often right after, they were potty trained for good.
My oldest potty trained himself immediately after we brought his baby brother home from the hospital. He, as young as he was, KNEW that was one way for him to get positive attention. Every time he went in the potty, we made a big deal out of him being the BIG brother, not like his baby brother going in his diaper, and it worked! Hopefully, you'll find the same thing!
Have you heard about introducing your oldest to his new sibling in the hospital to make him feel special?!?! We got some GREAT advice and used it for our oldest boys when the 2nd and 3rd came, and it worked GREAT! Let me know if you'd like to hear it.