If he was a bit dehydrated that would account for the weight loss. You would be surprised as how quickly they can loose weight through fluid loss, especially if they have a fever and aren't taking in fluids through the day. I think you will find that once he is well and eating and drinking normally again, that the weight he lost will come back on. As for his overall weight I really wouldn't worry. My DS is 2.5yrs in 2weeks and he is only just 13.5 kg, maybe 14kg now. But like your little boy he is super active and isn't the best eater in the world, he's just a toddler kwim? But if he doesn't gain that weight back he lost from being ill, then maybe just keep an eye on his food intake for a while just to make sure that he is eating enough and maybe slip in a few high calorie things like butter, cream and stuff like that to boost him a little.