Worms can come from vegetables or grass. It’d be great if you can get us a photo! If it is pinworms, they wont be visible on the poop after some time, they dry out quickly. If the white things stsy visible after say 30-60 min, its sth else. Rabbit pinworms as a rule do no harm. Signs of a bad infestation that requires vet attention for deworming are weight loss together with a disproportionally big belly and loss of fur lustre. With a heavy infestation you shouldn’t deworm yourself, since mass death of the worms will cause toxins to be released into the blood stream.

Many buns coexist without problems with pinworms.

A lighter infestation you can deworm yourself with fenbendazole (panacur).

If he isn’t pooping much that could be cause for concern. Can you offer him a big pile of fresh hay and some wet greens to see if that helps?

Agree with Bam about the worms. They can be treated by the vet, but I’d be more concerned about him not pooping as much as normal.

It’s a very nice drawing!  Here’s a picture from Medirabbit, of rabbit pinworms. The red arrows point to the worms:

Is he still not pooping as much as he should?

It sounds like pinworms, and depending on the level of infestation, pinworms will cause GI upset (that can then lead to GI stasis). In terms of how they get it, pinworms can be asymptomatic for a while, so it’s difficult to actually identify how your rabbit got it as it could have been a while ago, but is not expressing itself until now. I would prioritize getting the diagnosis confirmed and treated by a rabbit-savvy vet, as treating it will likely help your rabbit get through the GI distress which is decreasing his poops (as well as ascertain if him not pooping is linked to something separate).

Pin worms are very common and not usually a cause for concern. Sometimes though, they can get out of control and there will be too many of them. In this scenario, the bunny will experience GI upset. If the worms are affecting his eating/pooping, then they should be treated. I’m a little surprised that the vet would say that without seeing the poop. FYI – If costs are a concern, the treatment is actually pretty inexpensive.