If he's completely lost his appetite, just getting calories in to him is the priority, so forgetting about healthy eating for a bit and just going with anything he will eat - biscuits, chocolate, ice cream, what ever.
Nourishing drinks are good as pp have mentioned, as it's often easier to eat than drink - if he likes milk, that's great, use full fat milk to make any milkshakes or milky drinks, you can fortify the milk further by adding 4tbsp of skim milk powder per pint of milk. Milkshake drinks like build up can also be good. Otherwise smoothies and lucozade are good options.
Things that are easy to eat can be good, so snacks things like cheese and biscuits, mini pork pie or sausage rolls, thick and creamy yogurts or any dessert pots like little trifles, mousses etc, nuts and seeds, flapjack or cereal bars.
It's such a worry when kids aren't eating, hopefully he'll pick up soon, but contacting the GP sounds good too.