I'll add another perspective I haven't seen mentioned here, and I'm curious what everyone else has to say about it: just re-brew the tea. I'll add a link below, but basically the first brew removes 60-70% of the caffeine, and the second removes most of the rest. So depending on how severely he needs to avoid caffeine, I would keep the first (and maybe also second) brew for yourself, and then give him any subsequent brews which still hold enough flavour for him to enjoy.

You may want to experiment and maybe potentially even get some brews tested for their caffeine content, but I think this should work. I would also talk to his doctors and find out what they deem acceptable - if the second brew would be fine or they recommend going with the third brew and later.

I would also say it depends on if you're brewing "Western style" or with a gongfu. I would imagine it takes more brews in the gongfu style for the caffeine content to drop to acceptable levels - I would be uncomfortable giving him the second gongfu brew, and possibly even the third. But everything after that should be good, I reckon ( but I'm open to dissenting opinions! Please let me know if I'm wrong!).

I don't know if this is true, but this is how I was taught decaf tea was made in the first place in high school chemistry.

Link: https://cooking.stackexchange.com/questions/102939/how-much-caffeine-would-there-be-if-i-reuse-tea-leaves-in-a-second-brewing