Dd and I both breastfed our babies and both read up on the topic in whatever sources were available at the time (more for her, of course, with the Internet). While I understand that some women struggle with BF (I did at first), I also believe that sometimes it's due to lack of information, lack of support from family and friends, or pressure by well-meaning people to introduce formula too soon.
Unfortunately, as I understand it, the more the new mum relies on a bottle, the less her milk supply builds up = failure. Apparently, a supplementary bottle has worked out in some cases, according to posts here. But if all is going well, I wouldn't suggest it as yet, Op.
By "going well," of course, I mean, as others have said, if baby continues to gain weight ok, and also if he's wetting enough nappies (is the rule still 6 a day?. Also, as long as dd's CFS doesn't appear to be getting worse (but see that article I linked earlier). If there are any serious problems, then, of course, dd may have to supplement with or switch to a bottle - the main thing is that baby gets fed - but so far, so good, it seems, even though I understand you're worried.
Oh, of course, if dd would just prefer to switch to formula, then fine. There shouldn't be any pressure either way, imo.
This too shall pass.