good idea on hummous dip. If he likes it, you could also make an avocado dip - blitz it in the food processor with some garlic paste and tahini and water or lemon juice.
I used to make DS2 (v underweight as a young child due to refusal to eat) a very healthy cake - instead of wheat flour I used ground almonds and sometimes a bit of soy flour. Lots of butter, honey and brown ugar and then any flavouring he liked at the time. I used loads of eggs but you couldn't do that. You could make banana and almond flour cookies with butter and sugar and flavouring, then drizzle them with melted chocolate. (the banana acts as the binder, like egg would.) He might eat those.
Our dietician's advice was - as long as he is eating fresh stuff and you add vitamins to his drinks, and as long as you can get protein into him somehow, then don't worry too much if the bulk of calories come from less 'healthy' foods. It's more important that he gets enough calories than that every calorie is from a healthy source.
Have you tried nut butters? I used to make tiny postage stamp sized sandwiches with wholemeal bread, cream cheese and nut butter. It meant that even if he only ate a tiny amount, there were lots of calories in each bite.
Home made ice cream is good too. Put whipped double cream, vanilla essence, a bit of icing sugar and some frozen black cherries into the blender and blitz until they are a thick puree, then put them back in the freezer until they firm up to the consistency of ice cream. A small scoop of that would add lots of calories. Don't over feed the rich stuff as that might put him off it for good, but a single scoop of full fat cream berry ice cream and one nut and banana cookie is actually pretty nutritious, apart from the sugar.