You have a really good looking boy, and no he is not fat.
If I had to guess though, I would think he his not a pure bred. I have never heard of a boxer weighing in at 105 lbs at 10 months, also the last pic his muzzle appears longer than what you would typically see with a purebred.
Either way he is a really good looking boy in real good shape.
That dog is not fat. When you look down from the top of him you should see his waist ribs should be felt easily but not standing out. Be very careful with him at this stage of growing joint injuries are a concern especially with his size.