It's hard to say because there are so many different kinds of back pain, but I find doing anything for too long (sitting, standing, lying) will make it worse.

Can he have a little aspirin every couple of hours as a mild anti-inflammatory? That'll help reduce the shooting pains.

Maybe he can be allocated a seat in the aisle so he can get up and walk around and stretch regularly without disturbing the other passengers.

No alcohol, sleeping pills or anything that's going to encourage him to sleep for a long time in the same position.

The thermacare products look good, or even a wheat bag that the air crew can pop in the microwave for him every now and then.

Hmmm, that's about it. I feel sorry for him.