Are you doing them up tight enough? If they are a bit loose wee may be able to escape through gaps.
I wonder whether he may be doing large fast wees and it can't absorb through the fleece fast enough. You may be able to find out by putting something absorbent in between him and the fleece (a folded up flannel or something like that), I haven't tried it myself, just an idea.
Can't think of anything else that hasn't already been suggested, but if I do I will be back