GPs seem to be trawling through their lists, to identify patients most at risk, very, very slowly indeed.
When I was undergoing chemotherapy (and therefore had almost no immunity from anything), I never received the type of letter your hubby has now received. Months later though, when my immune system had started to get back to normal, someone finally decided that my name should be on the list and I got my first such letter. Since them, I keep getting them from time to time. (I've never taken the slightest notice of them anyway. For example, I've continued doing my own shopping but at times when the shops are very quiet. e.g. I started off using a local corner shop at 5am occasionally and then switched to shopping in Morrisons but only during the last half hour of trading).
It seems that your hubby's GP, or someone else with access to his medical records, has finally noticed that he should have been included on the 'shielding' lists about a year ago! You never know, they might eventually add your own name!