1) The land owners insurance--almost certainly not; there is no basis for liability, since the owner did nothing wrong; he had no knowledge of what your son and friends, who were trespassing, were doing.2) Your medical insurance--yes, it should pay for medical costs; that's what it is there for. If your medical insurance covers, let it pick up the tab and then don't worry about how else might be liable to repay the insurer--the insurer will take care of that.3) His friend's parents' homeowners--the injury did not occur on their land and was not caused by any affirmative act or negligence of theirs, from what your write; it seems unlikely.4) Your motor vehicle insurance--check the policy to see whether it would cover this situation. What it says in the policy and coverage declarations will determine.