I agree, just a growing boy! Sometimes it seems like there is no end to their appetite. Take him for a yearly physical, make sure he has healthy choices- other wise, sounds like you have a healthy growing boy on your hands!
Most likely just a growing boy. I have two sons myself and wow, my grocery bill!! And as they get older, expect this to get worse. A round 12 plus when puberty hits . . . oh my. Unless there is something that is causing a red flag to his doctor, I wouldn't worry. Is he really active? My boys are in sports and have been for years. They have always been run and play kind of kids (although they can veg out with screen time too, sigh). But when they are burning high amounts of calories with their activities while growing and especially if he is tall for his age, that explains the difficulty putting on weight. Does he go for an annual well check visit with the doctor? Most kids do and at that time, they measure them and tell you what percentile they are in. I have one son who is very tall and has always been in the higher percentile for height. And one who is not very tall and has been medium. And both are in lower percentiles for weight. Both healthy. Both growing on their curves and no red flags to the doctor. So, as long as this is the case, it is what it is. It's hard on you now because at 7, you probably still get all his food. Mine are a bit older and they can grab some food on their own. We have a fruit bowl on the counter and washed fruit in a dish in the frig they are always welcome to 'grab'. We have a box (Sam's club size) of granola bars and nut bars that they can also grab. They do normally ask m about other things before eating them. But lunch meat or peanut butter and jelly that they can make themselves, and hey, I'm not above ramen noodles as they can now boil water. But try to keep eating in a pattern. My boys eat regular meals and then have snacks and I do try to avoid constant eating all day long. They have a mid morning snack (it's summer and they are home), afternoon snack and evening snack post dinner. My older son has now started practices for his cross country team . . . 7 am every morning, 5 to 12 miles a day running. Has to go on an empty stomach. He now eats pretty much a full meal like pasta and chicken before bed to have energy in the morning. Ugh. That's getting old fast but he's handling it on his own, more and more. He is making a plate from dinner to have around 8 pm.
Long way of saying that I feel your pain. I'd say this is normal if he is super active or has a high metabolism and unless there are red flags from his doctor, I'd not worry. Try to think of easy ways to keep him fed so you don't go crazy. And I've stressed with my boys healthy eating because at some point, the high metabolism and massive activity will stop and then their food choices could catch up to them. good luck