I've always had irregular periods, and what's worse I can't remember my last period.
I'm a first time single new mommy to be. . my pronounced due date is July 5th 2011
Last person I was with was my ex... I can recall having unprotected sex with him
Oct.17th 2010... is he the dad?

Only a DNA test can answer that question.

If you had sex within a short period with more than one man, then as J_9 said, only way to know for sure would be to have a DNA test done after baby is born.

Were you with anyone shortly before your ex?

I will assume that there are at least one other person who you had sex with in and around the same time.

Due dates are estimated, not exact, so if you had sex with more than one person, within a close time frame, you will just have to do a DNA test to prove

I agree, if you have only had sex with your ex within a couple of months time frame either side of your due date then he would be the dad but if there is another male partner close to the time you slept with your ex then you will need a dna test. But remember it is important to let every party involved know what is going on and to be prepared for when the baby is born