Dogs can have the instinct to mate no matter the age..regardless, this was unintentional, Heidi.
Guest, If this wasn't ordinary behavior by him and since the bitch wasn't in heat, please contact your vet and discuss this situation and "blood" with him/her. He's 10 years old.. It could be a sign of dementia or something else not right in his system. He probably needs a check-up at the very least.
regards,
toodles