You've asked your boyfriend if he slept with this particular woman, 5 times, each time he's said no and has become defensive. It's not any wonder he's defensive, because you clearly don't believe the answer he's giving you!
If he's telling the truth and his brother slept with this woman, and not him, then it's entirely possible it's his brother who is the Dad to this woman's son. Of course there would be a resemblance to your boyfriend, because he shares family genetics with his brother.
You can see a resemblance between the woman's son and your partner but in actual fact, there might not be any. It could be that's what you see, but no one else does.
Has it occurred to you that even if your boyfriend had slept with this woman 20 years ago, that he might not be aware this young man is even his?? Or if your boyfriend's brother slept with her, he might not be aware he's the Dad?? Neither man, may not be hiding anything!
You need to let the subject go completely. Your boyfriend has given you an answer (more than once!). You need to accept the answer and move forwards or decide you don't believe him and end the relationship.