K.B.
With my daughter, I was able to use both her 3rd birthday and the upcoming activities as incentives. She was signed up for a day camp with her older brother and his friend, and she knew she had to use the potty to go. I had told her that in order to be a part of it she had to be three, which got her extra excited for her birthday, and potty-trained, so she connected the two in her head. She was quickly trained right after her birthday, after months of spotty success. I know girls are easier than boys, so it might be a little harder with your son, but be honest with him and let him know he's in control of being able to participate in the fun, older stuff.