It could be either but is more likely to be your boyfriend.
Day 1 of your cycle was September 2. Normally you will ovulate between days 11 and 16.
You had sex on day 13 with "another guy." You then had sex on days 14 and 15 with your boyfriend. Sperm can survive about 5 days so IF, big IF, there was any sperm from "another guy" then he could be the father.
No test can pinpoint the exact date of conception - NONE. It's all an approximation, sorry Doc. It can take several days for fertilization/implantation to occur.
To be sure, you're going to need a paternity test. Otherwise there can always be some doubt as to whom the father is.
Hope it helps.