L.A.
Hang in there, it happens! My mentor when I started teaching parent ed. was a licensed family counselor as well as a university early childhood education instructor and I still remember well the advice she shared about this issue: the less fuss you make about it, the quicker it will pass.
Just put him back in pull-ups, and don't give it too much attention. Meanwhile, be sure to give him lots of positive attention at other times. As you've already heard, it's totally normal to have these little set backs when the new baby comes.
It may also help to talk to him about how important he is to you and about how the ways you take care of him have changed since he was a little baby. It can't hurt to spell it out that you'll still be there for him, even when he doesn't need you to change his pants anymore.