So far as you can tell at this point, either guy. You will need to get an ultrasound in two weeks to know more. It is simply too soon for you to be able to get an answer, but this does not mean you should ignore timelines. When you are 6-7 weeks from the first day of your last period, you should have an ultrasound. Don't do it sooner and don't do it later. (Sooner, and the embryo might not show enough detail to help you. Later, and there begins to be a margin for error that removes the usefulness of the information the ultrasound will provide.)
When you get the ultrasound, ask your doctor not to use the first day of your last period in the assessment of the conception date of the baby. If this is too hard for the doctor to understand, just tell him your cycles are really irregular, and you would like an estimated due date that is based only on the crown-to-rump length and other developmental markers of the embryo, not on the first day of your last period. When you get the estimated due date from the ultrasound, take that date home and put it into a conception calculator online, or else just count back 266 days on a calendar. That will give you an estimated conception date.
Don't do this in or after your 8th week, it won't give you the precision you need. (By your 12th week, using an ultrasound for estimating conception can be +/- 7 days off because of different growth rates of babies, and by your 40th week it can be +/- 3 weeks.)
If the ultrasound gives you a date right around January 30, you still won't know, because living sperm from both guys was in your body then. But if it points to January 25, it will suggest your boyfriend. Did you sleep with your boyfriend again later than January 25, or was that the only time?
The 25th was the only time.