I don't think you are a bad mom, like PP said, it is your judgement call.
I guess my only thought is, if, God forbid, something bad happened, even if it wasn't because he was on his tummy, would you always wonder?
I personally wouldn't touch allowing DD on her tummy until she could roll herself. Even then, my ped said to put her on her back and if she rolled, she rolled, but not to start her on her tummy.
Also, incidences of SIDS have decreased since you, your sister (and I ) were put on our tummies to sleep. There are many things they know more about now than when we were kids. They think it is because of the "Back to sleep" campaign to place infants on their backs.