baby keeps vomiting formula milk

Hi, i have been feeding my baby with my breastmilk for 6 months plus. Once or twice, i ever tried to feed him formula. But he doesnt like it, he will gag. If he does drink it, he will vomit the whole formula after 30mins plus.. i have tried nestle lactogen and natureone, same thing, he will gag. I mix formula with breastmilk, same thing he knows it's not the same, so he gags.. i only tried like 1 to 2 days. But poor thing he will vomit it, so i don't dare to try until he likes it.. any advice? It doesn't help that now, my breastmilk supply is low.. i'm trying to increase it back.. of no other way, i will just have to keep trying to save my breastmilk