D.W.
Our pediatrician's best advice was to potty train them when they're ready, not when we want them to be.
Our son was past his 3rd birthday, but when he got it, he got it and has had few accidents and no regression.
Boys don't have as acute of a concept of when their bladders are full as girls at the same age - so, they tend to be older when potty training (according to our pediatrician who has 3 boys ages 5 and younger).
I'd encourage him without pushing. Our 3.5 year-old nephew is locked in a battle of the wills with his parents. They've finally thrown in the towel out of frustration until he's showing more signs of readiness.
Our neighbor's son was much closer to 4 and would run around the neighborhood naked last summer until he was ready to do it.
Good luck. I hope the frustration ends soon for you both!