So my son turns 1 month old tomorrow, and he's still not back up to his birth weight, and I am…

So my son turns 1 month old tomorrow, and he's still not back up to his birth weight, and I am breastfeeding. Did anybody else have this problem? Would it help if I added formula? My doctor is aware of this but his appointment isn't until next week. I am a desperate mommy and I don't want to wait another day knowing that he isn't growing.