Should I take finasteride or evening primrose oil for hair loss?

You should switch to an organic shampoo. You can also consider a natural DHT blocker like Saw Palmetto. There are also medications like Finasteride (a DHT Blocker) and Minoxidil (Rogaine) that can be very helpful in maintaining you existing hair, helping to prevent future loss and even regrow some hair. Before taking any medications you should discuss this with your general physician.

Primrose oil does not have the research saying it helps prevent hair loss that Finasteride has. There's a possibility it may help you but it likely will not give you the results that you're looking for, especially considering that it sounds like your hair loss is very aggressive. 

Primerose oil capsule does not help to slow down  hair loss. Most probably you have a AGA .You should better to start minoxidil to stop hair loss and visit a hair expert.

There are hundreds of treatments for men with balding. Some help and many do not help. I would suggest you review your options again with your dermatologist. Evening primrose will not slow balding in a male who developed balding at age 16.

Your biggest enemy is the progressive thinning leading to a balding pattern that is not evident at this point in time. The degree of thinning that you are experiencing would suggest that you should see a doctor who most likely will get Bulk Measurements of your hair to determine the degree of the thinning that you are presently having. If the diagnosis is early genetic hair loss (the most common cause of hair loss in young men), then the best treatment would be the drug finasteride which is highly effective in men of your age and may not only stop the hair loss but possibly reverse it. If you are developing a more advanced balding pattern the best test to get will determine the Bulk Measurements of your hair and this will show, over a one-year time frame, how effective the treatment will be. Once you know this information, then you and your doctor need to develop a MASTER PLAN which will define what you need to do over the years to come