11 month old losing weight!?

My boy has been slowly dropping down his curve since two months, which is fine because he was still always gaining and growing as he should but over the last two months his appetite has really gone down which worries me. He has never been a big fan of solids or milk really and he's super active and literally doesn't stop moving for one second. He drinks on average 26 oz formula per day and I offer him three meals as well. Sometimes he will eat, especially if it's pasta or anything with yoghurt but mostly he likes to play with it and make a mess. If he drinks a bit less milk he usually eats more and vice versa but there is no way I can get him to eat or drink more than what he wants. I'm trying my hardest to trust him to know how much he needs but when I weighed him today as he turned 11 months he actually lost half a pound over the last month. I know it's because he's super active and he's simply not taking in enough calories but I feel like I offer him enough but he simply doesn't want it. His pediatrician is never concerned since he's fine otherwise. Anyone else been through this? Should I just stop worrying??