Hello and Welcome!
Don't just look at how many cups he's eating (b/c each food has a different amount of calories per cup) but look at how many calories he's eating – he seems to be eating on the low end for a Boxer puppy with a high metabolism. I’m not saying encourage weight gain but I think you can increase his food a little. The back of each bag of food will give you guidelines as to how much to feed but you have to adjust it to fit you dog’s needs - each dog is different.
I checked Nutro Lamb and Rice, it has 340 calories per cup which means he’s getting 1020 calories a day (without treats of course). Owen is almost 9 months, weighs 57 pounds and is eating about 1800 calories a day (not all kibble though, about 1200 of those calories is kibble, the rest is homecooking). Any less and every rib was very visible, he was way too skinny. Owen’s still on the skinny side but that’s normal for his age and I don’t want to go over 1800 calories – you don’t want to overfeed. Pitcairn and Strombeck’s books have charts in it which tell you where your pup should be at what age regarding calories and all I remember is according to those charts Owen is on the high end at 1800 calories - it took some time to see exactly how much worked best for him. I'm not saying feed your Boxer 1800 calories, that may be too much for him. I will check tonight to see what the range is and let you know but I think 1020 is on the low end. How much does your Boxer weigh?
I wouldn’t necessarily feed him more dry kibble, maybe a little, but try adding some canned food at each meal. Or try a kibble that has more calories per cup – one example, Canidae has 466 calories per cup so you could still feed 3 cups per day but be at 1400 calories.
Being skinny is normal for his age though. Sometimes they don’t finishing filling out until about 2 years of age.