I also have a Mastiff/GS mix (yours looks like a dane though). He's a little more than four but is a rescue and was chained outside for his first three and a half years. He's just as sweet as can be--and smart--but getting aggressive with other dogs, especially males. He's 140 lbs so I'm looking for any advice folks have. I've gotten rid of all toys which look like animals--as he shakes them when he's playing and seems not to differentiate small dogs from small toys.