I'm 21 years old with patchy facial hair and wondering, will it stay like this forever?

Facial hair or having a beard is a genetic trait.  There are no medication to grow a beard. One option is facial hair transplant

It will typically change over time.  Some men just don't have that distribution of hair on the face where others have a significant amount.

Facial hair growth is usually genetic. You should wait until you are 25 to consider a hair transplant, for there may be more growth. Additionally you could try using minoxidil 2% over the areas where you wish to see growth and it can help. Please consider consulting your doctor for discussing your options. 

Growing beard hair is genetic. Some people grow beard at a later age or won’t grow at all. It all depends on your genes. 

Many young men want to accelerate their beard growth and ask about it. It takes age (genetic triggers which occur in family lines) so if you want to find out when your beard will grow out, you need to find out when your father or grandfather started to grow their beard. Hormones should have kicked in nicely by the time a man reaches 18 but he must wait out his genetics as hormone alone are not responsible for the onset of beard grown. The drug minoxidil may help.  Beard transplants can be done when the young man is a bit older (over 25) if the beard has not grown in yet