Just how small is your little Boxer? Three months is still very much a baby. At six months your dog will be roughly half the size it will be when it is fully mature. And remember, a Boxer isn't an incredibly large dog.
Just make sure your pup's getting good quality food with plenty of quality calories for it's activity level. We give our pups calf milk replacer as well as Innova puppy feed. It supports bone strength and growth, which is important since they do grow so fast.
Do check with your vet to make sure there aren't any underlying problems, like thyroid deficiencies, kidney problems or anything of that nature.