How to treat head acne on a grown man?

Scalp Acne. You are probably referring to so called: scalp acne. Doxycycline has been used with some success only. It happens usualy in males and might resolve after age 40 spontaneously. It could be frustrating especially with males losing hair and acne lesions easily visible. Treatment need few months for effect.
Sebaceous glands. Acne can occur anywhere there are sebaceous glands (oil glands) in the skin. Web md is a great resource to learn more about acne. Skin hygiene with soap and water followed by benzol peroxide or 2% salicylic acid products are over the counter options. An evaluation by a dermatologist can help develop a long term plan. Resistant acne can also be treated with antibiotics and some laser therapy.