Milk allergy? So yesterday we took my son to see his pediatrician to change his formula bc he still…

Milk allergy?

So yesterday we took my son to see his pediatrician to change his formula bc he still hasn't pooped since Sunday. He was on gentlease bc of his gas but I think his gas was caused by bad latch on his bottle and since then we've found one he latches better on. Anyways our pediatrician pushed nutramigen, it's a hypoallergenic formula and she thinks it's the lactose that is causing the constipation. The thing is that's his only symptom, he's not fussy or anything, of course he's just uncomfortable bc he needs to poop. I guess I was wondering if we can go rogue a little and try reguline or if it is possible it's the lactose and should just trust his dr. The new stuff is SO much more expensive (if he needs it well pay) and if he doesn't have a milk allergy I don't really want to cut it out of his diet. What would you do?