What's the best way to thicken that is thinning due to mpb?

Thinning Hair -- Rogaine, Propecia, Viviscal, Progesterone, PRPIf you have thinning hair you really need an evaluation to see if it is genetic or from another cause like stress or autoimmune. If it is determined to be genetic there are many options and traditional treatments like rogaine foam and propecia pills can be used. I have great success in my practice using injectable progesterone and prp to the scalp. Postmenopausal women also benefit from spironolactone. I suggest seeing a dermatologist with expertise in hair loss. Best, Dr. Emer.

There are medications for Male Pattern Balding.  See a doctor get an exam and treatment options.  MPB is genetic.  You can't stop it, but there are medications to slow down the hair loss or sometime regrow hair.

This is best treated in young men with the drug finasteride, which may reverse the hair loss and bring back the thickness of your hair as the returning hair will be normal, not thin. You must see a doctor and get the diagnosis made first. 

it depends on the degree of your hair loss. If it is not too advanced you can start a medical regiment routine but if it is advanced you can consider hair transplant.