Maybe give him the "bland diet" for a day or two (my awesome recipe hahaha is below) and then transition him to the new grain free food? Anytime these guys have tummy upset that's resulted in "the squirts", that's what I do. Until it settles down to a semi solid poo at least. The liquid poo shouldn't go on for very long or there could be a dehydration risk. Is the pup drinking normally? I don't know what to say about the blood thing. I'd be back in that vet's office - but I'm a nut.
Recipe: 2 cups cooked white rice + 1 can white meat chicken with juice - mix. They love it.