My 10-week-old kitten has chronic loose stools. Is KMR safe long-term?

He could have a sensitive stomach or a malabsorption problem. Unfortunately KMR isn’t a well balanced meal for him to consume after the weaning age (3-5 weeks of age), so it’s best to keep him in a high quality kitten food such as Royal Canin, Hill’s Science diet, Evo, Eukanuba or California natural by Natura. Your vet can run a test to check for malabsorption. This is something that can be treated.
I would also recommend trying a probiotic for a few weeks to see if this makes a difference. Over the counter Culturelle is comparable to a lot of the pet formulated probiotics we carry in the clinic. Adding in 1-2 tsp of canned pumpkin (not the pie filling), can also help solidify his stools.
If all else fails then it’s best to go back to your vet to have a pcr fecal screening test, which is a little more in depth to check for bacterial infections that would not be detectable with just an in-hospital fecal exam.
I hope this helps and best wishes moving forward with your pet Charlie and his loose stool. Take care.