Generally speaking, your neighbor has the right to 'dump' water on his property wherever he wants assuming he DIDN'T alter the terrain in any way along the path the water NOW, or use to take. If he dumped water at Point X in his yard and it naturally flows to Neighbor B, he can do it. If he dumps water at Point B and it naturally flows to you, he can do that. If you take any of the 2 Points and water no longer does what it use to do, you may have rights.
Here's an example ... If your neighbor put in a flower bed that now PERMITS water to flow onto your property where it use to, say, flow onto a different property, you may have a case because the natural flow was 'altered' (it now goes to YOU.) This may be a case where either the flower bed has to be removed and the ground restored to its 'natural' condition, or, the water needs to be re-diverted to where it use to flow, or diverted to some place else like the street. To be sure of your rights, check with you local municipality.
I have to ask, why is he refusing to help? What would he have to do to 'fix' this? And would if you don't have any rights, what would it take for YOU to do the work for him? What is the set-up between your 2 houses??? Please respond ...
PS: My neighbor on the 'uphill' side of me actually put in a flower bed along my fence and it now KEEPS water in his yard from a downspout where it use to flow to me. That's OK. BUT, if here were to remove the flower bed now, he'd have to put the landscape BACK to its original, natural setting. Of course I'd get the water again but he didn't do any man-made alterations to the ground that promoted the flow to me. (If he went and dug a trench to my fence, I'd have a word with hime.)