Hi there, my ds is very thin also, 4yo, bottom centile. Not weighed him lately but last time we did he was around 11kg.
He has been a fussy water but is getting better all the time. The more we relax the better he gets
It's hard when they're like this but if all the signs are that he is well I wouldn't worry too much. You say he eats a varied diet and is energetic, that's great.
I would maybe just do some research in to foods a that are calorie dense, as in you don't have to eat a lot of them to get a lot of calories. Things like peanuts and rasins for instance. Advocado is a good one.
We tend to use full fat spreadable butter on all ds's sandwiches and give him lots of cheese type fillings a soft cheese, normal cheese, cheese spreads. Also things like peanut butter etc. he ears fruit and veg also obviously. Things like milkshake or smoothies are good too, just full fat milk with a bit of milkshake powder. He also has cakey snacks and puddings etc.
Tbh though all this doesn't make a massive difference as he's so energetic he must burn a lot of it off! Some people are just made this way, lucky him I say. Dh and I were skinny kids but you wouldn't know that now
I think the main thing is not to fret about it, they pick up on it even when you think try don't. Try not I discuss any of it in front of him too. These are things I've had to learn myself!