Rusfox
According to birth control calendar method I learned many years ago:
To say approximately, the safe days are 8 days from third day of menstruation, and 7 days before first day of next menstruation.(assuming your period is 28 -30 days (calculated from first day of menstruation til first day of the next menstruation)).
Everything in between those days is red zone when you can be ovulating and get pregnant.
In your case the red zone was from 3rd July til 17 July.
So I would assume chances are 70% that the baby is your boyfriends.