I do sympathise, I make my daughter pack lunch and find the sandwhiches have been squeezed into a tight ball inside tin foil to disguise fact they have hardly been touched, 3 crisps have been eaten but chocolate cereal bars eaten! Half cheese string consumed, barely enough to keep her alive I think!
She does love cucumber though, hardly high in calories.
All I can suggest is just really try and find out exactly what he does like and just give him those only. There are a few things my daughter will eat (very few) but a few things are healthy and so I will make sure she gets those on the plate to encourage her to eat other things on plate also kind of mix em in with other stuff.
I mean you say he likes sausages whilst not healthiest option he is young so he can take it and it is good protein and high calorie to get the weight up, you could slip him vitamin/mineral supplement if he is fussy about veg/fruit. Maybe just try to get him used to 1 new thing a week?
I know its a tough 1. My only consolation is the fact that I know I was exactly the same and am now complete opposite love eating out anywhere. I really grew out of it when I reached about 15 so I'm thinking my daughter might too.
If it really is very bad then I would definitely speak to a professional maybe as I can only comment on my own experiences with my daughter.
At the moment I would talk to him and just play it down a little maybe so it doesnt become a big thing in his mind.
hope this helps