T.S.
I second exactly what PP said. It would not be at all unusually for him to have gained only one pound (or no weight) from May to October. It isn't like when they were tiny and packing on the lbs every could of months. To set your mind at ease, take a look at average weights of 6, 7, and 8 year old boys. You'll see they differ very little. Your son will likely only gain a couple of pounds a year moving forward.
He did NOT weigh 59 lbs in October if he was 48 lbs in May. If he did THAT would have been cause for concern. The nurse who put it in his chart WROTE IT WRONG. He probably IS looking thinner, but thinner doesn't equal weight loss for growing kids.
You probably DO want to up his caloric intake and make sure he's getting plenty of protein and limiting the empty junk. My skinny minny son and I both like to have a smoothy in the morning AFTER regular breakfast. It's an easy way to get in any extra couple hundred calories. I add a scoop of soy protein (you can use another protein depending on what you think is healthier for boys/humans), yogurt, frozen berries, a banana, and oj. He drinks about 8oz of that before school and and another something similar in the evening before dinner.