Patching jeans question

This is post only. I'm sorry. I have a baby blanket I'm almost done with and I've been making catnip toys, so I'll be posting pics of them soon.

This is a very simple and stupid question. My son has a hole in the knee of his jeans, and I thought I'd patch them with part of a shirt that my youngest has outgrown, Do I need to fuse some backing to the material before sewing it onto the jeans? I know it would make the patch last longer, but is this something I really *need* to do? I always hated that stuff on the inside of my clothes when I was a kid, and it bugs my son too. These are just play jeans that I thought I'd spruce up a bit.

Thanks!