The homeowner may be responsible if you cannot find out who actually did the damage. However, if the accident occurred on private property, versus the car being parked on the road in front of the house, this may alter the person's liability. If the homeowners will not tell you who hit the vehicle (or if they don't know and you don't otherwise learn who allegedly did it), then pursue a claim against the homeowners. If the property damage is minimal, then retaining a lawyer or suing them without much evidence or a police report may prove futile. You should have filed a police report and let the police do their own investigation. That is likely why your car insurance won't cover it, as accidents such as this where there is no police report are often considered scams or suspicious. You can still make a police report even if it's been a week or a month after the accident.

Also, how does your son know his car was hit while at that house? If he saw it when he went to leave, did he ask anyone at that time, or look at any other cars to see if there was damage (although the car had probably already left)?

This is one of those life lessons where acting intelligently at the time is often the best bet - such as immediately contacting the police. Unfortunately, I had the same situation growing up - neighbors had a huge bash and someone totalled my parents' vehicle. Nobody would talk or fess up. My parents had to pay the deductible and make an insurance claim. Not much you can do sometimes without a lot of evidence, eyewitnesses, or honesty.