I'm a 49yo man and have had a panty fetish since I was 12. I think if you were to go buy him a few pair of really nice ones, put them in a box and gift wrap them, then just casually say "here, I bought you something", then hand him the box, he will be pleasantly surprised. But don't stop there. Encourage him to try them on for you. I think this lead to some of the best sex you two have ever had. If you give him a spanking, it would be a real bonus. Let me know how it turns out.