If he's got runny poop, it could be a sign of gastrointestinal problems and I would check his diet. The basics are greens, unlimited hay, optional pellets, lots of water, fruit for treats occansionally (You probably already know that and I doubt that's the problem) Yeah, it could be the probiotic powder for sure, some side effects of it are pooping, I think that would be what's causing it and not GI problems, I'm sure you know about rabbits.
Call your vet and ask him/her, that's what I would do. To clean your bunny's bottom, not entirely sure, just take a wet cloth and gently clean him up? You could pet him and just give him a few treats to reduce stress. I hope your rabbit gets better, such a cute bun.
Site to look at if you want, you can google the side effects of probiotics too, but just ask your vet
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https://www.myrecipes.com/how-to/co...n, in fact, make,not to stimulate your bowels.
I'm not an expert, and I'm sure others will have great answers as well. Good luck!
Take care and best wishes
~ Remy The Rabbit
