Sorry I'm only just coming back to this, but thankyou all so so much for taking the time to respond, there's some really brilliant advice here I really appreciate it 
I can't scroll back once I've started typing so can't reply to posters individually so just going to do a general response to some questions and comments
DS is actually nearly 7, not just turned 6 so it'll take more than a couple of pounds to get him to healthy unfortunately
Lots of posters have suggested hidden calories which is what we try to do for the most part, but he's not a fan of 'wet' foods so it's difficult, although I do make him eat these occasionally anyway (with a bit of bribery)
He would eat flapjacks/milkshakes etc like lots of people suggested, thing is we also have a 5 year old who is a perfectly healthy normal weight so it would seem I was rewarding DS with goodies for not eating his meals, which wouldn't be fair on her
To the PP who asked how he is with food, he is totally uninterested in food
He sees it as an inconvenience and only ever eats when I tell him it's time to eat and everytime its met with "eurrghhh do I have to?" If left to his own devices he wouldn't eat all day, food just does nothing for him, he's not excited by it at all
He also eats painfully slowly and gets "full" (or bored) very quickly
Feeding him is quite the saga really
I've taken on board people suggesting 'easier' foods like softer fruits and cereal bars and it's actually working, he's eats more of them and eats them much quicker so thankyou (this one seems so obvious now
)
Oh and he hates bananas
Think I covered everything there, thankyou all again, I'm open to anymore suggestions