Usually, players will breed or train an entirely new team of Pokemon to use. So, largely, the pokemon your son already has isn't really going to be helpful. It does depend on what moves his Pokemon know or can learn.
In particular, his selection of Pokemon isn't really helpful. Volcarona is a threat in singles, but in the Doubles style of battle that they use at Regionals, its typing renders it weak to a wide variety of common attacks. Its strong attacking moves - Heat Wave and Bug Buzz are quite good - are nice, so it's not unsalvageable.
If we go a different route, Audino can learn a move called Trick Room, which causes slow Pokemon to go first. Audino also has access to the useful Heal Pulse and Safeguard, and the Healer ability (if your son's has it), which help it support its team by keeping them healthy and preventing status conditions.
Wailord can learn Water Spout, and that itself just makes it a massive threat, but it's quite slow. Give it an item like Life Orb and use Trick Room, and it'll do huge damage with Water Spout and go first thanks to the field effect. Samurott can similarly benefit, but not quite as effectively.
VGC/Doubles isn't my strong suit so this is kind of just a glance at where you might start. Hopefully some others can give you some more concrete ideas based on this!