Hi Firestorm, we had a similar issue with our puppy on Skinners puppy food and have since moved her onto Burns dry and her poo is less frequent and more solid now. I read quite a few recommendations for Burns being good for high energy dogs, and as well as the improvement in her toileting, she doesn't get as hyper after meals as she did with the Skinners.
It is worrying when they have toilet issues, but I think runny tummies can be pretty common with active puppies. Even on the Burns, Ivy will have a loose poo when she's running around outside like a nutter (strangely, only after lunch and only when other people stop to talk to us...)
If you're seeing blood in the poo, I would take him back to the vets as that's often a sign of a stomach bug or bacterial infection. If it's a blockage, I believe there would be other symptoms. If you want to put your mind at ease, you could ask the vet for an x-ray? They can be very pricey though. They offered us an x-ray at our vets when we thought Ivy swallowed a cufflink, we didn't go for it in the end as my husband just got his metal detector out instead (no cufflink)!