It is possible that you would be liable for his injuries under one or both of the following theories:1) Your son committed an intentional tort--he deliberately pushed the other boy; this would be offset by the other boy's role in agreeing to play this dangerous game, but it's not a given that the other boy's participation will provide a defense.2) Seven-year olds probably should *not* be allowed to play, in fairly large numbers (6 children) on a trampoline without adult supervision; you may be liable for "negligent supervision" in not doing what a reasonable parent would have done under that situation.