My son is spitting up too much

My son is 5 months old and since he was a month old I started him on formula. I first started him with Enfamil the yellow can. He would spit up almost the entire feeding I’d give him and it look almost like rotten milk. I asked the doctor about it he suggested smaller ounces of formula so I tried that and it didn’t help. I changed his formula to similac sensitive and the same problem happened I recently changed it to similac spit up and he just spit up again. I’m making him a appointment to see the doctor in the morning but I don’t understand what’s going on. Is it normal? I’m afraid he isn’t gaining weight.