First off, nuts aren’t a must in anyone’s nutritional problem so no void to fill as long as you are eating healthy proteins, fish and fats. But if he’s simply looking for alternatives for snacking, I suggest seeds like pumpkin and pepita seeds. Also using tahini (sesame) in place of nut butters. I love a wrap with a little tahini, honey and banana for a sweet snack.
Also roasted chickpeas and wasabi peas are great alternatives!