Not enough information to even speculate. Need to know -- At the time, were your periods absolutely like clockwork and came exactly every 28 days?
-- Did you have an early ultrasound in the pregnancy (approximately 6 or 7 weeks from your first day of your last period)? If so, what due date did they give you at that time, from the measurements of the embryo seen on the ultrasound?
-- Did the baby come on the due date? Does the child look like your ex?
If your doctor took your reported first day of last period as the start of your pregnancy time period (which is how doctors count out pregnancy), you would have been told a due date of August 2, as you were told. That's just the mechanical count like you would do on a calendar or little wheel, starting on the first day of the last period. But you would not have been told by the doctor that you were in your eighth week on December 11. He or she would have said you were about 6 weeks 4 days into your pregnancy.
So, I am trying to determine where you got the number of "8 weeks pregnant" and how you associated that with December 11. Did the actual doctor who was looking at your actual ultrasound say 8 weeks, or did you go home and figure out 8 weeks? If the doctor said you were 8 weeks pregnant on December 11, that would mean he was disregarding your report that your period began on October 26, and it would be surprising if he would tell you August 2 as a due date. A doctor might indeed tell you a number of weeks different than expected from the first day of your last period, but that would only be if he or she saw something on the ultrasound that contradicted your report of when your last period had begun, and if that had happened, he or she would probably tell you a changed due date.
So, the question is, who did the analysis that said "8 weeks," you or the doctor? And if it was the doctor, did he do so based on what he saw on the ultrasound? If he changed your count to 8 weeks based on what he saw on the ultrasound, did he also change your due date at that time?
I guess while I'm asking things, I should also ask, did the due date of August 2 come from the doctor, or was that something you worked out yourself by counting from the first day of your last period?
When the baby was born on July 24, did anyone comment about the difference between the due date of August 2 and the size and development of the baby?
After all this time has passed, it might be hard for you to remember what parts of the story the doctor said and what parts you decided yourself. What I'm saying is that October 26 and August 2 match, but they don't mean anything if you simply worked them out by yourself some time on a conception calculator, and the "8 weeks on December 11" doesn't match them at all. The only parts of the story to use in assessing which guy is the dad are the parts that came out of the mouth of your doctor once he or she read the ultrasound. If you can't remember after all of this time what was said by the doctor once he or she looked at your ultrasound (not just based on your report of when your last period was) and what parts you figured out by yourself, there probably won't be any way to try to puzzle out which guy is the dad except the DNA test.
Sounds like the answer is, it could be either guy, and you need to do that DNA test.