How heavy is the book bag

We kind of tricked my son into weighing himself! I have been getting comments that he looks slimmer. He has been carrying around a super heavy bookbag and to had him weigh with it and without it.. the bag weighs over 20 pounds! Well, anyways, before school started in late August he was 175 lbs. Now he is 178 pounds. That is 3 pound gain in about a month. I am so unsure if that is a good thing or bad. He is getting taller.. I can't say if he has gotten taller in a month ect.. He finally has a PE class but otherwise isn't getting much physical activity in. I do make him come on some walks in homework allows it. Next month, he goes back to the doctor for a vacine so we will see what is then.. Also, is weighing in once a month ( say that cause its been a month) a really good way to track if he has lost or gained? Do boys get the added water weight? ect. I cook breakfast for him Monday thru Friday! I don't count his calories but aim for at least 500 calories. He has a late lunch. We are not in control of what he eats for lunch. Dinner is usually pretty healthy. We don't count that either but sometimes I have a rough idea because I am counting mine and I don't cook seperate meals. We do allow some treats but not a lot. Sometimes, though, I do think we could do better with this. I do notice my habits are better if he is around. I won't over do it on sweets if he is around to see it.. but if he isn't home and I hang out around a big selection of sweets it could be bad. Well, anyways, just wanted to do a quick update on his weight.. even though I am not sure if that is a good thing. Like I said. People have made comments about how he is looking good.