I've been a vegetarian for 31 years and have no adverse affects, in fact I pretty healthy all things considered.
It would be lovely if you and your husband could find a little room in your lives to support your son. It might indeed be a fad, but nevertheless let him try!
Lots of people have made some good suggestions on here already and you never know hubby might even enjoy a couple of new menus here and there. Perhaps you can cook a big veggie curry tomorrow and compensate by going all meat the next day while your son has a pizza or something?
Try not to let him live off cheese sandwiches; beans on toast is OK occasionally, but when you're desperate for a pn evening meal, go for a jacket potato with beans and side salad, or have some frozen indian meals ready for him.
Sandwich filling ideas.
Marmite and watercress
Feta and olives
Feta and beetroot
Peanut butter
Humous and cos lettuce
Falafel salad