Best thing to do is to confirm what’s going on by seeing a dermatologist, who will also be able to help you with your options/treatment.
That being said, your doctors concerns about finasteride are probably related to your body not having reached maturity. Because your body requires a specific hormonal recipe to finish developing, it’s a bad idea to take finasteride or dutasteride prior to the completion of said recipe. Most men aren’t there until their 20s and while some choose to go ahead with treatment anyway, they do so at higher risk of side effects and developmental issues.
Blood tests can help a doctor evaluate and explain your individual situation so that you can make whatever choice you are comfortable with. It’s a heavy drug and it permanently changes your hormonal balance; you need to be absolutely sure you need it before you start and to, if possible, hold off until your body fully matures.
To be clear, you can take finasteride prior to this point, and many have done so without issue, but the risks are higher and you need to be sure you are comfortable with that before you take it.