If the vet wants you to feed a prescription diet, it shouldn't be mixed with anything. That said, I'm really not sure that he needs it. Maybe the Orijen just isn't agreeing iwth him and he needs something else or maybe he just isn't feeling well because of something else. I'd probably go back to the old kibble, wait a few days and try again. If he keeps spitting up on the old kibble, you wiill know that he just isn't feeling well. If he stops, you will know it is the Orijen. You could try to reintroduce again or find something else.
I don't think D/D will hurt for 10 days and if you are worried about gastritis, I'd go for it but a dog spitting up twice doesn't mean they should be on a prescription food (my opinion only).
