Doberman refuses to play and doesn’t like toys. Won’t even play fetch.

Hey! I got my Red doberman 2 years and 3 months ago as a puppy. I’ve raised him singlehandedly since then. He was neutered at 1 year 3 months old. At some point around a year and a half of age. He just didn’t enjoy playing with toys. He likes to chew on tennis balls until the pop but usually all he does is wraps up a blanket, and chews (more like pulses his jaw) on it for up to an hour. Although, he has done that with blankets almost his entire life. He is the most gentle and submissive dog but loves to play with other dogs of all sizes. He is fully sociallized and has never been aggressive towards any dog or human, but barks at every squirel he sees. The only thing he seems to enjoy doing is going for walks or going to the dog park to run and play with other dogs. He eats blue buffalo, he has never once tasted human food, and he gets hours of vigourous exercise every day (he is at home 6-8 hours alone every day but gets a walk in the morning and 2 hours of walks and the dog park when i get home from work) I do not use any physical force or raise my voice as a way of training him; But when I throw a ball or a blanket and say “go get it” he puts his ears back, his tail between his legs, and walks the oher direction or goes and lays down in a corner? He will always come when I say “come”. But is almost afraid to do anything related to playing. He brings toys to me often (when he isn’t sleeping or howling) and then drops them as soon as I grab ahold of them? Tug of war has always been equal and I’ve always been obliged to hold his bones from time to time. Yet he just refuses to do anything but play with other dogs, sleep, or chew on a toy alone until it is out of commision. Even when I give him bones, he usually just licks them for awhile and ditches them. I’ve even attempted soaking them in hot water. For some reason he is just always discontent and gets scared of played fetch regardless of my level of enthusiasm. After awhile of trying I usually just walk away. I do not know what else to try!! Someone please give your insight!?!?!?!