Try him on something like boiled/ cooked chicken until his poos firm up. Something really bland with no additives. I did this with Prestoo when he was wee and had a bad bum. It took a little while (2 days) but he got better. After that add a good high meat content food and not dry - if he's got the runs he will be getting dehydrated and dry food won't help that at all. Make sure he's drinking too. If you get really worried take him back to the vets.

EDIT : Presto was the runt of the litter and so so wee. He had a bad dose of the runs on and off for a few weeks and we always thought he would be tiny - he's HUGE now. So don't worry about it stunting his growth I doubt it will.

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