Formula recommendations

Hi, mommies. I've been wanting to change my son's formula since about a month and a half after he was born. He's now 4 months and the formula he's drinking is still no good for him. He would throw up a lot since he was about a week old and he would just wriggle around and never stayed still while eating. He arches his back and grunts while he eats. He does that til this day. I was told that a month wasn't enough time to see if the formula was right or not, but it kept happening. He would throw up a lot. Not just a little spit up here and there, more like an ounce and a half or so (I'd say) every single feeding. At first he was having bowel movements several times a day, then it went down to once a day, and now it's every 3 or 4 days and it's solid and so thick. I have to take his diaper off and put his little legs close together and his knees up to his stomach to help him poop and it looks painful because it's really thick. He has a very hard time going #2. I brought this issue up to his pediatrician when he was a month old and he said it was OK for it to happen. That as long as he was gaining weight, it didn't matter. I brought it up again at 2 months old and he said the same thing, followed by "we don't know how much milk is in his stomach, so it's normal for that to happen." He still vomits about thr same amount almost every feeding and now he has a very hard time going poop. He also hasn't been eating as much for the past 4 days. He's teething and it's very itchy. He was up to 6 ounces every 4 to 5 hours and wouldn't ask for a bottle at night. Now he's down to 4 ounces or sometimes even 3 and a half and won't take more than that. That worries me a lot. Especially because this formula is already messing with his little stomach, so I want to change it myself. I thought the doctor had to give the "OK" but I've been told he doesn't have to. Also I buy my own formula, so I don't need a prescription from him or anything. He's drinking similac pro-advance (gray top) but I don't know which would be best for him. I'm thinking the similac (orange top) maybe?