Unless he is emiciated, doubling his weight at his age isn't likely. It's possible, though. You'll be very surprised what a great diet can do. So you'll just have to wait and see. You can usually tell by looking at their frame what weight would be healthy. Since he's still growing, he may very well end up at 5 lbs.
He does appear long in relation right now, but he's at that age. Most go through that. It'll all even out.
My chance looked exactly like Forrest before he was neutered. He matured kinda slow due to his size. But neutering him helped get some weight on him. At one point he was as round as he is tall.
We had to cut his food back, and now he's at a healthy weight.