Don't focus on the dates. 25 weeks 4 days ago was August 8. Doctors base dates on your LMP.
You had sex on day 16 - and he pulled out.
You then had sex on day 17, 18, 19, and 20 with your boyfriend.
Timing wise it could be either. Sperm can survive about 5 days. Usually you'd ovulate between about days 11 and 16 of your cycle, or between August 17 and 23. The egg is viable for about 48 hours after ovulation.
Ejaculating, even outside your vagina, then putting his penis back in can lead to pregnancy. There would be semen (with sperm - not precum) in his urethra. Just inserting the penis would force this out and into your vagina.
Odds favor it being your boyfriends but there is ALWAYS the chance it was the first guy. The pull out method is unreliable and precum can contain sperm.
You'll need a DNA test to confirm paternity.
Good luck. Hope it helps.