We've supplemented from the beginning despite everyone telling me I made enough for my DD I didn't and she wasn't gaining. I would just keep at it, if your baby is hungry it will eat( you can try a syringe if that helps or you can put a tube on your nipple and place it in the formula bottle). Keep at it.
Is it the formula he won't take, or a bottle? You can try supplementing on the breast If it's the latter. Full disclosure: we supplemented on the breast for a while but both my LO and I found it frustrating rather than helpful but I hear it works for some folks. Good luck!
Re: formula supplementing
Keep at it.