Have you tried going up a size in the sposies? There have been times that I needed to use a larger size at night time than in the day.
Sometimes it is just one of those things. I have a little boy in my daycare that from about 2.5 until 3 he would soak himself from armpits to knees every day during a two hour nap. I still have no idea how he even produced that much urine. He was PL'd for daytime so I would make sure he peed and then we put on a fresh diaper right before he went to sleep, and he always woke up soaked. He just grew out of it eventually.
Is there a time in the evening that he is consuming a lot of liquid? Maybe trying to play around the timing of that would help. My DD often leaks during nights were she has had a bath (she consumes a lot of bath water - yech, I know) and I find if I keep her up for ten more minutes she will have a post-bath pee, I can change her diaper and she won't have any leaks. But if I put her right to bed she will have that same pee and sit in it all night and the diaper can't hold much more.