Mix Enfamil Formulas?

My son is 2 1/2 months old and were still trying to figure out which formula is the best (he also has colic) he was on Enfamil Premium Newborn, but then the doctor had us switch to Gentlease because he said it would be easier on his stomach, it was for a little bit but his colic was way out of hand so we tried the Enfamil for Colic and he didnt like it at all, so now our doctor gave us these drops to help him with his stomach and we tried gripe water, they helped for a few weeks but now its like they dont do anything for him anymore, so now the doctor says we should try Enfamil Prosobee (for sensitive tummy) I was just wondering how many days do I mix the formulas so it will be easier on his stomach than just giving him the straight soy?!? Thank you.