Advice on adjusting to new formula?

We were using the Parents Choice brand of Gentlease and switched to the actual Enfamil brand of Gentlease because that's what WIC will be giving us and we wanted to be consistent. My son started having more bowel movements and way more gas. The gas doesn't seem to make him uncomfortable but it's just louder and we notice it more. He also has a diaper rash that I think is attributed to the increase in stools. He hasn't spit up or anything. Do you think he is adjusting to the formula and we should give it a few more days or should we switch again? I don't really have the money to try tons of formula. When should I call his doctor about this? It's been about 4 days. His stools aren't really loose, though they were at first and he is wetting as normal.