I've done a lot of grill gifts. It depends on the type of grill and how they use it. But some various products:
Grill Rescue Brush
Meater thermometer
Thermapens
Ash Vacuum
Custom grill cover
Custom branding iron
Also, a lot of the grillers in my life are deeply into sous vide as well, all in the pursuit of the perfect meat. If he doesn't have one of those yet, it is something else to consider.
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