Julie W,
I think this is a common problem, if you look around you will find many threads on this!
I too struggle with this (is it a boy thing?!) and have found that the leaking is worse if he sleeps on his side. Now that my lo is older (6.5 mths) and no longer swaddled, he prefers to nap on his side and fall asleep on his side at bedtime. I generally leave him for his naps (not long enough for a leak problem) but at night once he's asleep I will gently roll him onto his back.
As for how to prevent the leakage, here's what I have found works best (bearing in mind that my lo sleeps through the night):
1. Slkather on the nappy cream
2. Insert a nappy doubler into his bedtime nappy (like a big sanitary napkin, in fact these would do just as well). Be careful you don't place it so the top of the doubler is at the top of the nappy - I have found this just wicks the wee right up into his clothing.
3. Put on another nappy (next size up) over the top of the first nappy
4. Put plastic pants on (I use the type which are shaped and have velcro fastenings at the front, not the pull up kind)
HTH!