In order of importance:
1- Is he fit for surgery? This needs evaluation by an internist. There may be other issues such as heart or lung conditions that need to be tackled before surgery.
2- Are the back problems related to the abdomen? If the back problem is independent of the abdominal girth, he may benefit more from addressing that problem first, as the increased mobility and comfort will help him solve some of the weight related problems.
For this I would have him evaluated by a Physical Medicine and Rehabilitation specialist (Physiatrist, not to be confused with psychiatrist). My wife has back problems and the Physiatrist has been a godsend.
4- Is the fat inside the abdomen? As others have mentioned, if the evaluation by the plastic surgeon is that the fat is intraabdominal, then the solution is weight control.
5- If the back problem is related to the abdomen, the answer may not necessarily be surgery. Remember, Tummy Tuck is a contouring technique, NOT a weight loss technique.
6- If the back problems are a barrier to exercise, consider aquatic therapy or exercises. In the water, the weight is not borne by the back, and a lot of muscle groups are engaged. This may be a wonderful way of achieving several of his goals: exercise, weight loss, increased muscle tone to support the back, etc.