Hi
I was a large person (230 lbs) and had knee problems. My PT had me ride a bike to correct it. I can say though I biked between 7 and 20 miles a day, i did not lose much weight. I did lose inches. I found that i had to start slow. I felt lucky to go a mile. But i was persistant and now no amount of biking bothers me. Even if I stop biking for a month or two.
I can tell you his butt will be soar from riding for the first week or two. but it will eventually go away.
As far as the best kind of bike for him it would depend on what kind of biking he wants to do. Street bikes are great for paved paths. they are light weight and go at a good clip. Mountain bikes are for dirt. Many men and women like them. Then there is the comfort bike. this one is a street/mountain bike. It has a comfortable seat--the best seat on any bike for soar bums.
I hope this helps you. You may want to get a bike yourself. It really is a lot of fun once you get used to biking again. And there are so many beautiful paths everywhere.