Sick from formula? Help :(

Bree • Mom of two sweet babies - Avery 👶🏻💙 3.20.16 and Emmy 👶🏻💗 8.18.18 @strong_like_emmy IG <— follow her journey
My baby was born preemie by 7 weeks so since the first day, he's been on a combo of my milk and formula. Similac neo sure to be specific. I pump and pump and can barely make enough in 24 hours for one feeding, (low supply the whole 8 weeks due to me not taking care of myself when I was recovering from my c section and he was in NICU for 2 weeks but that's another issue) and so at this point, he is mostly formula fed. 
After almost every feeding, he has spit up. I think it's in normal parameters, but, just to see if it was formula related we tried switching him to Emfamil AR for spit up and he had a bad reaction to it. WE THINK. 
3 days of Enfamil later, he's got hard, round lumps of poop (yup, constipated) and he's been projectile visiting the last 3 feedings, big violent amounts. Not spit up, definitely vomit. Looks like undigestrd huge lumps of powder. Way bigger than the "cottage cheese" that sometimes shows up in spit up.
Has anyone ran into this? What have you done to help your baby throughout? He's so fussy right now and I feel terrible. He's got to be in pain in his tummy and he looks like he's trying to poop and can't :(. 
I threw away the rest of the enfamil and gave him similac the last 2 feedings and will have him only take his similac from now on, but in the meantime, how do I help his upset tummy, vomiting and constipation? 
PS- I KNOW he feels sick, he is literally never fussy. Any advice is appreciated!