Any yea, somebody here is going to say he deserves it. Whatever. The relative has been there for me when I needed help, and it would be nice to keep him around a while longer.
p.s. 13 year old unvaxxed relative in the same household got covid at the same time. Has the sniffles and is fine.
Rheumatoid arthritis is a common side effect of any vaccine. I hope he feels better soon. Once he is over the flu you may want to look into the autoimmune diet to help with the joints.