Hi Saku. I'm afraid I might not be much help (and always check with your vet first), but if your cat is having problems with keeping food down then maybe dry food isn't the way to go. It's harder to digest and requires your cat to drink more as wet food contains more mositure.
Also if you check here at Pet Forums or search online you'll find some pretty bad info on Iams as a company.
Royal Canin is good cat food so I would try the sensitive stuff you've talked about in your post. As to how much to give concerning the pouches, two to two and a half a day should be fine. If he eats little but more often then maybe try half a pouch at a time, so he still gets the four portions a day.
Also, Renal food may be an option. It's for cats with kidney problems, but it's easier to digest than regular cat food. My parents had two cats on a permanent Renal diet, Tigger (sadly past on now) had thyroid and kidney problems and Abbey (still with us) who's just older, around 14-15 yrs.
I've got my fingers crossed for Bailey and if any of this helps then great.
Good luck
