I also agree with neutering him. Granted Thor isn't a "bully breed", he's a chihuahua, we didn't have him neutered till he was a year old, we were slow about it-that's the only reason. It's been almost a year since he was fixed and we haven't noticed any change in his activity and he's lost 1/2 a pound He weighs 5 pounds now. Also no change in his guarding abilities, in fact he has more guts now!!!

Just portion the food and exercise him. We have a female doberman who isn't spayed, but only because I want to show her in AKC and they can't be spayed. I can't imagine her changing her attitude just because of a spay. It's all instincts, she's knows what to do. I think it will be the same with your male.

My parents have a great dane and a wire hair fox terrier, both males, both neutered. The dane is very protective of the house still-he was a rescue at 5 years old. They also free feed and only the fox terrier is slightly overweight, but he's 11 years old so you have to give it to him. This is the only time he's ever been overweight.