C.W.
Yes! Please follow up with the rheumatologist. Also when he gets like this, try giving him a warm bath and let him just play for a little bit. After the bath, lotion his little body up if he'll let you and give him a little massage. That may help decrease the pain just a little bit. (I'm 23 and have arthritis throughout my whole body and this is what I have to do A LOT)
If it is some sort of arthritis, the heat will definitely help the little guy. I'm not sure exactly what it could be, but i hope that they'll be able to help you figure it out very soon!