Do you live near a Tractor Supply or another farm supply store? I would run there, if you can, and get some dewormer. I use Ivermectin, but some people use Panacur or Safeguard. The 1.87% Ivermectin paste is marketed for horses and comes in a tube for about $4.
My rabbits don't willingly take it, so you have to squish a small, pea-size amount into their mouth (or put on their lips and wait until they lick it up.) Dose him again in 7-10 days. Then wait another 7-10 days and dose again.
If the deworming helped before, the problem is likely parasites. But as mentioned, you have to treat more than once to get them in each stage of their lifecycle.
Hopefully that will help, and it's a cheaper solution than running to the vet, if that's not an option right now.