Pressure can cause a problem after as little as 20 minutes, although the damage may not be visible for a while.
Sheepskin liners are often used and in the past were thought to relieve pressure, but they don't actually relieve pressure. So it might feel more comfortable, but is not effective at pressure reduction.
In the US, you would need a referral from your doctor to a nurse specialist in skin care (In the hospital where I work, it is called a Wound, Ostomy and Continence Nurse).
Here's a
web site that has a forum that you might be able to get some assistance from.
I'm not sure what effect the increased pressure in the airplane would have on either air-filled or gel filled wheelchair seat cushions, but I think they might swell a bit and might not be as good at pressure reduction as a ground level. Memory foam wouldn't suffer from that problem.
A few things I can think of would be to change the seat back position periodically. Doing that changes the place on his bottom that is getting the most pressure. Another thing to change pressure would be to raise one or both of his feet up periodically. That would also change the pressure point.
Another thing (that maybe the nursee could help you avoid) is shear injury while moving him from the wheelchair to the aisle chair and then to the seat. You will have limited space for moving around and that makes it harder to avoid shear while you move him.
I haven't seen any information about relieving pressure on the websites of US airlines, but British airline may have more info, since anyone treveling form the UK to the US would obviously be looking at a long trip.