It is possible for the baby to be your boyfriend's, if you are saying that you got the 5 weeks count from a doctor using an ultrasound. If you just got it from an app, that is a slightly different question. Did you sleep with anyone else after your boyfriend?
We know just about as well as you. Dna testing is the most accurate. When did you have sex with the other guy?
If you had sex with someone else around the time you conceived, yeah.