His hernia can come back! I heard that when they do hernia surgery there is always a possibility that it can return.
Have the hernia repaired and then exercise. My husband had a hernia and went regularly to the gym with the thought of reducing it. I had a fit and told him he would make the hernia worse. It did, and he was hospitalized for a major repair. I don't know if it works the same with an umbilical hernia, but common sense tells me that you should first consult a doctor.
I'm sorry, I'm not a doctor, so I can't answer your question. I would doubt that they can just "pop" the hernia back into place, but I know of people who have had surgery to repair hernias. You will need to talk to your doctor about that. Regarding exercises that are safe for it -- I really can't answer that either because I don't know how much pressure it can take. I would think that walking would be one thing you might be able to do, but again, you'd have to talk to your doctor. I'd hate to tell you something, only to have it make things worse instead of better.
Regarding the hypothyroidism -- there are a lot of us on this forum who with being overweight due to that, including myself, so I *can* help you out there. The first thing I'd want to know if whether or not you are on med and if your thyroid levels are good. Since this is the weight loss forum, please feel free to send me a PM regarding your thyroid issues AND I also recommend that you check out the thyroid forum here on MedHelp, as there are a lot of very knowledgeable people with tons of info.
Can you give us some idea of what kind of diet you eat and what your lifestyle is like so we can help come up with ideas for you?