On nov 16 I cheated on my boyfreind.. im now pregnant and my baby's due September 12 culd it be the other guys baby?

Oh yes. It can definitely be the other guy's baby.

The ONLY way to prove positive paternity would be a DNA test AFTER the birth.

Maybe it belong to other guy.. but think about that month if you and your boyfriend had sex.. if yes der still possibility that child is to your boyfriend but if you want to be sure dna will do help you

There is no way to be sure an exact date of conception, also you did not provide much information, Due date, how was it figured, from last period, or by ultrasound.
What dates did you have sex with the other person or persons ?

Date of conception is give or take a couple weeks at best, so in general there will be no way to know that is safe for the baby, until the child is born and a DNA test is done.