Sometimes rabbits need a longer spell on hay only if they have poopy butt. That your vet said he was fine just means that his health isn’t in any immediate danger by poopy butt. It doesn’t mean you shouldn’t try to do something about it, it’s bothersome for both the bun and the hoomins =)
Poopy butt means there’s a slight disruption in the microbial balance in the gut. Hay is the best rememedy, but it can take up to 3 months before the tummy normalizes. Some buns are extra prone to poopy butt and will have relapses now and then. You can try adding a probiotic as Dana says. Benebac is used by many of our members and it’s available in many parts of the world. There are other brands as well, I used a British brand for my poopy butt-prone bun Bam.
You can try different hays, in case he has a sensitivity to a certain hay. There are actually rabbits that are allergic to timothy! I myself am allergic to timothy so I try to feed other grass hays. Rabbits can eat and thrive on any grass hay that’s good for horses. The easiest available types are probably orchard and oat.
If your bun is a good hay eater, it’s risk free to switch him to a hay only diet, he’ll just eat more of the hay. If he is not a good hay eater, you should taper down his other food until you see that he actually eats enough hay. Otherwise the bun might eat too little or not be able to extract enough nutrients from the hay it eats (for this the gut must have enough of the appropriate microbes in the gut), and thus it will starve – which is very, very dangerous for a bun.
You can do a hair cut around the but if a lot of poop get stuck there. Beware not to cut too close to the skin. A vet or vet tech can do a full sanitary shave if needed. You can give your bun butt baths when he is particularly poppy, or you can do dry baths by massaging cornstarch into the poopy clumps and then comb or brush them out. The safest way to give butt bath is in a wash basin or other small-ish container that you place on the floor in case the bun jumps out. You just need an inch or two of lukewarm water. Make sure you dry him properly after so he doesnt run around with a wet butt and gets cold.
I don’t know if you have read this, but it’s a good article so I’m linking to it, other readers of this thread can perhaps benefit from it even if you yourself have read it already:
http//rabbit.org/intermittent-soft-cecotropes-in-rabbits/
Part of this article is about true diarrhea, this is not what your bun has.