Can anyone help,my dog was castrated about 6 weeks ago and i was wondering how long the sperm stays in his system for.

It has been known for a dog to sire pups up to 6 months after being castrated. Is there any particular reason you are asking?

If it is because you have an in season bitch, then please still keep them apart as yes, there is a risk he could get her pregnant. Better still, have you got a friend/family member he could stay with, or kennel him.

Yes i have got a bitch that has just come into season,but we are keeping them apart,i just don,t want any accidents as they are mother and son.

Ok - well, yes definately keep them apart - not easy a lot of accidents do happen as people are not aware of how determined a dog and bitch can be. If you have anyone he can stay with, then better (and less stressful) for him to go on a little 'holiday'. If the worst does happen, don't forget that you can get a mismate injection, not to be relied on, but should be used as a last resort.