Everyone responds differently, but weight gain and edema are known side effects, especially longterm, so I think it's very possible.

There could be other things going on as well, e.g. diabetes and other conditions can cause edema.

I've been told, when gabapentin was prescribed for me, that the weight gain was at least partly due to the side effect of increased appetite. I don't take the gabapentin anymore, but I still have a constant battle with weight because I have back & mobility issues as well, and the extra weight only exacerbates the back issues in a vicious cycle. I follow a high protein/low carbohydrate diet that's worked better for me than others.

If it's edema in his arms, I would think you could tell by the same pressure-pitting(?) I have it in my feet, and that's another constant battle, but pressure and exercise (walking) definitely work better than not, even with a cane.

Doc