Have you had him seen by a vet recently, courtenay.m? Even taking in a stool sample would be helpful. Regardless, if you suspect worms, he needs some sort diagnosis so the proper meds can be prescribed. OTC dewormers don't work well, so the visit to the vet is well worth the money.
If he tests out clean for worms, the vet may have other avenues to explore as to why he's not putting on any weight.
Good luck with your little one! When you figure out what's going on, I hope you post some pics of Hemi for us! We love pics