E.M.
Is he potty trained at nighttime?
We dealt with this--to an extent-- with one of my twins. One was wearing underwear all night, while his brother was still in diapers for bedtime. With DS2 (the one that would stall going to bed) what ultimately worked for us was making a clear rule around bedtime: aside from going potty prior to laying down for bed, he got ONE opportunity to go to the bathroom, once they were down for the night. After that...well, if he *genuinely* needed to go (and most of the time, he didn't--but was just stalling) then, he had a diaper on.
We always made an exception, for if he needed to go #2. But, if he's up more than once in the night, needing to pee, we draw the line there. His brother, unlike DS2, is a VERY light sleeper...and having one wake up the other all night long is a huge no-no for us.
Have you considered cutting out the naps, if possible? I know that's a hard thing to lose...but, it may be contributing to him not wanting to go to sleep as easily at night.