Portabella mushroom or any mushroom really, is a neat meat substitute. You get a meat like texture and it's way more manly then tofu, which I avoid like the plague.
I'd introduce a less meat alternative, assuming he eats mushrooms.
Shread up a steak, a cheap cut, and pan saute it with whatever else he'll eat (peppers
, onions, garlic, ect) Have him grill the portabellas and spoon in the sauted steak.
If he eats corn, even better. Soak corn, husk and all, in the sink with a liberal amount of salt. Place it on the grill and steam it in the husk.
The typical guy loves to grill.
If you're ever going to get him to look at a meal without meat as a meal, you've got to first show him how it's manly -- but just don't say it!