Health potion can be made by synthesizing berries that can be picked up in the forest where you can go from the beginning. There are recipes that you can learn as you progress through the missions, and some that you can discover by trying them yourself.
Ah, I just checked the required materials in the save data,
the last one is just a normal health potion (:
I think it would have been fine if you had Bandages + AntiBreeding Ointment + Spectrum + HealhPotion in your inventory. After that, I think that if you kill time by repeating sleep and waking up until the morning for a few days, and talking with Tank, Scar will recover. I think it's a good idea to save once you have all the materials just in case.