I would wait to buy something really special. Then every time you see or use that item, you would be reminded of him kindness. Or maybe split it up, if there is enough, between something special and some clearance fabric.
My M&FIL gave me a large gift cert from my LQS for my birthday. I am going to use it to buy the fabric for a pieced king-size duvet cover - as soon as I can make up my mind about what I want to do!!! I want to be able to think of them whenever I see it, which will, of course, be everyday since it's for our bed.
They will also see it when they some to visit because they often use the master bathroom off our bedroom when they are here (long story, too boring to post).