Best formula for beastfed babies?

I am in need of advice. My son is almost 8 months old was exclusively breastfed, until this past Monday he has been on formula. (He doctor told me I could, I don’t respond to the pump) I’m not sure what to do to help him though. He spits up a lot or will throw up once a day now. He never use to throw up or spit up. Now he does. He drank two once around 3:40am and the at 6:20 he woke up and I gave him a fresh bottle he only took a could sips and threw that up. I know he isn’t over eating. What could this be?? I can’t call the doctor until Monday. And he doesn’t throw up every time he drinks it. Only once or twice a day.
Thank you.