My suggestion would be let the etiquette books do the confronting for you. Grab a book or print a list of the grooms family responsibilities as far as $ goes and show it to him. It's his family so he should be the one talking to them about it. They should be asking him how much he needs, not making him ask them. There are very specific things they are supposed to pay for and even then it doesn't make a dent in the overall costs.

Good luck, I hope they contribute something!