I agree with all the above John, for all the reasons given.
When I had my first Cocker boy, Josh, many years ago, I was asked by somebody if Josh could be used on his Cocker bitch. This was in the days before DNA health testing so that didn't come into the equation. Josh had a good pedigree, was a handsome boy and had a lovely temperament and the thought of perhaps having a puppy by him was tempting. But when I thought about it I realised that it wasn't a good idea at all. I had no experience of dog breeding and I'd seen Josh's best friend Smartie, a Cocker the same age, completely change character with other dogs once he'd started to be used as a stud by his owner/breeder. Smartie and Josh used to play together but once Smartie had been used at stud he used to go for Josh and other male dogs and was always busy sniffing around bitches, spayed or unspayed. I'm not saying that all stud dogs become like that but it's a risk you'd run with Dennis.
You have a happy and sweet natured young dog in Dennis, who gets on well with your girls, so please just keep on enjoying all three of them as you do now
