It could be a slight case of swimbladder disorder. Yes, peas help this issue or so I hear. I know my female beta liked to eat them before she passed away. Don't feel bad about starving them. Fish can live off the fats in their liver for at least 5-7 days. Feedings should be only once daily. It is a good sign that he still eats, but like you said, you never know how old he was when you got him. I had a beta that I had for less than a year, and no signs of illness. He actually ate just fine and the water was fine too. Just died one day.
What other kind of fish do you have with him? They are supposed to be species only, but some people have had success with other fish. I recommend not to, but I guess it's a personal choice.
Do you have a water testing kit? If you do, do a test to let me know what your water parameters are. Also, how often do you do water changes?
To answer your question about if he dies while you are away, he should be removed immediately after death because otherwise whatever he had could infect the other fish (although I am 95% positive he doesn't have a contagious infection or disease). Also, the other fish could eat his body, which is also not good.
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Phishie

"An ounce of prevention is worth a pound of cure."

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