Re: 1st formula baby, horrible reflux, bleeding privates, help please!:(
I would guess that the constipation is just because of the quick switch and him spitting up again COULD be related to the constipation (not as much room?). I don't think you ever mentioned his age? If he's old enough, maybe some prunes or prune juice...something along those lines. I know a lot of people use Miralax as necessary too. When we switched, our son didn't poop for a few days and had us a little worried. He eventually did though and has been regular ever since.
I would stick it out for a good 2 weeks (unless he seems worse on it than the Nutramigen). It can just be tough on little ones switching to a whole new food. Hopefully he'll adjust soon. If not, give Neocate a try. Just like with anything else, different ones work for different babies!
Extended rear-facing, non-circ'ing mom to 3 boys - P (11), J (10), and E (4.5)