Generally speaking, a materialistic person isn’t worried about being perceived as materialistic, so take it easy on yourself.
You need to be honest with your friend and kindly express that once in a while you’d like him to pick something out for a memorable occasion that you can put in a keepsake box to look at and remember as the years tick by. Assure him that even though he’s not feeling sentimental right now, the day will come when he starts to enjoy an occasional trip down memory lane with you.
He’ll need some suggestions so reassure him that anything from a little snow globe to a fridge magnet are great memory triggers, but what matters the most is that he spends some time and uses his own imagination. To have fun finding something unique and heart felt. Cards are nice, but a stranger wrote them and thousands of people are getting the same one for the same occasion. How many people will get a snow globe for the anniversary of their first winter together or a magnet that looks like their puppy? You get the idea. So will he. Just be sure to gush with appreciation so he’ll continue.