Typically, there is no liability on a travel trailer unless you're a full-timer, and that's to cover someone getting hurt while it's parked and suing you - liabilities that the normal person's home insurance would kick in for. While being towed, the two vehicle's liability kicks in and covers damage the trailer causes in an accident, up to the limits of the policy.
To be sure, your son should first check with his insurance companies (auto and home) to be sure what's covered, if anything, with respect to the trailer. If it's lacking in something specific he wants protection for, then it's time to start shopping.
I'm not an insurance agent, so I won't say I am 100% correct, but the above is my understanding.
Rob - Solo Full Timer
2017 Winnebago Travato 59G
Former 2006 Mandalay 40E