Obviously as everyone else mentioned, talk to your vet.
But our vet usually recommends we put our dog on a white rice/ground beef diet when he's off. This is just easy to digest for them and still enough protein and such. Just enough for his system to reset if it is a stomach thing.
And for him going inside, it will require supervision, but it should be pretty obvious when your dog is about to go (ie. excessive sniffing, circling, etc.) so watch for that, force him outside, then praise him when he goes.
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