Most vets won't castrate before 6 months and many advise on waiting until later, a year or so, to give the dog time to mature physically and mentally. His growth plates will still be open for some months yet and castrating before they close can lead to plates closing at different rates, a dog with longer and lighter bones, and joint problems. Also, as already said, he will still be capable of siring puppies for some weeks after castration.
I've never had a dog and bitch together, but every time someone posts a question like yours on a forum there will be lots of people saying how difficult it is to prevent them from mating. It's not just that the dog will be desperate to get to the bitch, she will be desperate to get to him too. You won't be able to have them in the same room together, even under close supervision, and you may end up with two very unhappy, whiney and frustrated Cockers.
Friends of mine have Border Terriers and their male goes to stay with friends when their bitches come in season. They don't want to risk an accident and the male goes off his food and becomes extremely restless and whiney unless sent away.
If you were unlucky and they mated and Lottie became pregnant you'd then face another dilemma, because she's too young to have a litter, being physically and mentally immature herself.
