As long as he has no significant medical issues that would prevent it, riding a bike is as good for his health as any other form of exercise.
Having said that, the type of riding (where and how) will have a major impact on the type of bike to obtain. Ideally for new riders, a place to ride separated from traffic is the way to go. Rail Trails, Multi Use Paths, Parks or even around the block in a quiet neighborhood are a good way to start.