J.R.
Hi S.
I really dont think there is a big difference in formulas and that switching to similac will upset his tummy!
They are all pretty much the same stuff! At times if I didnt have formula but had some samples laying around from the hospital of enfamil (b/c my baby used similac) I would give her the enfamil and she was totally fine! So dont worry!!!!! I think the biggest change for a baby is when you stop breast feeding, if you ever did, and switch to formula. That seems to be when they have the hardest time adjusting tummy wise.
Good Luck!