Are you in the same reservation/pnr? If not joining the parties may be a good fiirst step? If your husband has no status and is on a separate reservation then (ignoring for now the medical issue) BA will require him to pay for a seat assignment (long story and lots of threads with pros and cons). It may be that that issue is getting in the way? And joining the reservations should enable himto benefit from your status and get over that hiccup.

Or is it that you are trying to get an upstairs seat to match yours, and perhaps no suitable seats remain free uostairs?

KF