It will raise him up a couple of inches. That doesn't seem like much, but there are two very important things to correct. First, he needs to have his feet reach the floor with minimal pressure on the back of his thighs. If the distance to the floor is even an inch or two too far, this pressure will reduce the circulation in his legs and his feet will swell. Swollen feet and ankles are very common when mobility is limited and we have to be very careful not to add to the problem.
Swollen legs are miserably uncomfortable and the damage to the circulation becomes permanent.
Second, it is very important that his arms are well supported by the armrests of the chair. Bill won't feel this happening at first, but in time it will make him uncomfortable, especially when lying on his side. We have a 1988 Plymouth Voyager Van with a lift and tie-down system for a power chair. Excellent condition, New battery and tires. We are in the Cleveland, Ohio area.