He needs just hay and water until his tummy settles. I'd suggest cold wter with a dash of boiled water added, to make it warm, and also regularly changed. I woudl also suggest pushing the hay, so every hour or so giving another fresh handful.
Then, you will need to introduce the pellets as you should normally, but without the change of the others.
So you'll need to give a minimal amount on day one and day two, and then on day three increase by a tiny bit, and then keep increasing every other day, until you are giving the amount you want to give.
Its never advisable for people to do anything suddenly with a rabbits diet because of the problems it can cause, like what you're seeeing. Anything needs to be sone slowly and gradually to minimise the risk to the gut.