Hi there and welcome. We have a breast feeding forum that you may find helpful.
My first child nursed until around 5 and a half months. I began supplementing with formula as I felt he needed it. My second son nursed until I finally said "okay, we have to stop this" and he answered back. (little joke . . . but he was embarrassingly old when I discontinued nursing him).
I will tell you that both boys are healthy and just fine with my son I nursed continuously with no formula being the kid with allergies and smaller than my other son.
This is my way of saying that I'd try to feed a good deal (I know it is tiring especially if he takes small amounts all the time instead of larger feedings in one sitting) and pump when you can to stimulate the flow and keep it going. If it doesn't work, then supplement with formula for as long as possible to keep the nursing going (it is such a wonderful bonding experience) but do not worry about any health implications to bottle feeding. Like I said, my son who was 100 percent formula at 5.5 months would probably be considered healthier than my other son who nursed for eons. Both are smart, well adjusted, great kids.
so, you can only do what you can do and try not to stress over it. For me, stress is worse than anything else because your baby can pick up on that and the bonding, lovely nurturing experience of nursing loses that quality. peace and luck to you (and enjoy your little one!!)