I'm going to second lanolin. It is edible but doesn't taste good, so kids are discouraged from licking their lips. I had terrible torn chapped lips last year and nothing was helping. I tried lanolin, and after a week, there was noticeable improvement. Put it on every night and every morning - and any other time you notice that your son's lips look particularly irritated. I now use it every time my lips (or skin elsewhere on the body, too) starts to get chapped to stop the damage in its tracks, and it's the only thing I've found that really, really works.
ETA: Vaseline is not edible. I am not comfortable putting something that should not be eaten on my kids' lips, personally. Carmex is illegal in Europe, so that settled that for me since they have tougher chemical laws there. Good luck.