Lottie is roughly the same weight but she is very small - we met another orange roan yesterday who was literally twice her size. My vet said for her personally, she could stand to put on up to a kilo but she wasn't worryingly underweight. As she's not all that keen on dog food (years of being fed roast dinner I suspect) it's hard to feed her up, kind of the opposite problem that you have, LOL! There's lots of good advice on here, so I'm sure once he's properly grown he'll settle into his ideal weight.