My cat is bald all over!

My male cat Chef is 3 1/2 years old. Up until this past September he was a pretty normal cat. Then I moved to a new place. I had moved four months prior to that move and he was scared, but never had this problem. I always keep my cats treated with flea medication. About a month or so after the move, I noticed he was balding around his stomach and on both sides. I asked my sister-in-law, who is a vet tech, what she thought. She said that sometimes it's fleas or sometimes cat pull their hair out due to stress (ie the moving) or worse case senario he has mange. I have never seen the cats with fleas. I have another cat, my girl cat, and she is not balding anywhere so I was pretty sure it wouldn't be mange because she would be bald too right? He doesn't have any sores. I love my cat, he's like my kid, and I don't want people to think I don't take care of him or that he's abused because he comes from a very loving home. I have never found clumps of hair anywhere and I have never seen him pulling out the hair. I just want to know what's wrong and how to fix it because he bald in quite a few spots especially underneath. Is there anyway to prevent this or help him grow back his hair?