Why Substitute?
Cooking is best done on the fly. Throw caution to the wind, skip the nuts.
If you skip the nuts I promise the food police won't come to get you.
Here is a tip for dealing with the Food Police - Just make sure that you feed them when they arrive and they will leave you alone.
Joking aside - there are many times when there is something that my wife does not like that is in a receipe and I simple omit that item and I create something unique for our house.
Some would say that for authentic pesto, you need pine nuts-some substitute wallnuts for ease. And while wallnuts are definitely nuts, pine nuts are actually seeds. I know of several people who are allergic to nuts and can still eat pine nuts (seeds). Do you know if you're brother can tolerate them? Good luck.