Hi Sarah,
Sorry to hear your Dad is hurting. We've been there. Along with pain meds, "Magic Mouthwash" is recommended. It's a solution he can swish around his mouth and swallow. It numbs things up nicely and gives you a window to get some food down. He should be rinsing with a solution of water (16oz), baking soda (1tsp), salt (1tsp) and glutamine (5g optional) 4-6 times a day. The key is to stay ahead of the pain. Don't let it get to the point that you're really hurting before you take it.
The HBO treatments you read about are typically used to treat healing issues with post treatment dental work.
Positive thoughts and prayers
"T"