There are a couple of things....
1) Since he's a boy, try making sure his penis is pointing down when you put the diaper on. I've notced a couple leaks every once and awhile at the top of LO's diaper, and this is usually to blame.
2) Double layer with the same size at night. This way your LO won't be swimming in the diaper but will still have extra protection.
3) Try an overnight diaper, like pampers 12 hour protection/overnight. They have higher absorbancy at the same size
4) Use the bigger size diaper and deal with LO being in a big diaper, just at night