Hey Liz,
I would also start it ASAP! DS and DD both at 3 months took to formula - DS Nutramigen and DD Alimentum - no problem. With DD I did the nutramigen as well, but she reacted to it, went back to BM and then Alimentum and she was not fazed about either both times.
HUGS. It is so frustrating how something one thinks is supposed to be natural for babes (drinking milk as they are hungry!) can be SO stressful!