If you really can't make anything to send him, you could make a donation to a worthy cause for him, like to PETA or the Humane Society, or to Slow Food (
www.slowfoodusa.org), etc. He sounds pretty conscientious, so I'm sure he would appreciate something like that.
Maybe you could make something for him at a cat-free friend's house? I second the stencil ideas-- those are always so simple and versatile! And i love the idea of a personalized yoga mat bag.
You could also send him a In-a-box type gift that is themed around local foods where you live-- you know, like more herbs/canned foods grown by you or other local people, local honey, regional recipes, etc.