I don't know, but I thought you'd like to know that you aren't the only one! My son will be 7 in 2 weeks and still in pullups. His 3yo brother is usually dry at night, and will wake up to go, but my older son sleeps like the dead and doesn't wake himself when he has to pee.
I look forward to seeing what responses you get because we've also been thinking that summer will be a good time to work on this. So far the only ideas i have are to limit liquids at bedtime (which we already do) and wake him up at 11:00 or so to pee, which we haven't tried yet.