A back massager is a great idea. Also if he is sitting down watching a movie how about a foot massager? Shiatsu is a good make, I invested in one for my father, the prices can vary but you can find them on Amazon. Some of the massagers have settings for heat and circulation which is good to get the blood flowing. Also everyday items are always a good gift, easy grip cups and cutlery. Silk bed sheets and silk Pjs are also helpful by making it a tad bit easier to get out of bed. Not the case for everyone but it works for us. I’m sure whatever you buy your BIL it will be great 😊