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how old is your girl?
Has she ever been in heat yet?
We have a wonderful expert on this site. Her name isXXXXX has five Mastiffs and is extremely experienced with this breed. I am sending her your question as she is best qualified to be of help to you.
Is that OK?
You're boy isn't going to like this, not one bit, but he can't have her this time around or even next time. Mastiffs mature very slowly...she is too immature mentally and physically to whelp and raise a litter, so do your best to keep them separated the whole time or you may be biting off more that you can chew with raising an entire litter yourself and risking losing your girl in the process.
Ok...The entire heat cycle lasts 3 weeks...between days 9-14 she is most fertile, but that does not mean she can not get pregnant on the 20th day. :) I know it must be frustrating for him..my male, in my profile, Bob,(as a pup) goes nutty when we have a girl in season. It will subside and he will go back to normal :) Hope this helps and good luck in the future :)