Hello and welcome to the forum. Thank you for your question but we are sorry that your brother has this going on. I highly recommend that he see a doctor as soon as possible to discuss this. It is hard to say what is going on. A few possibilities include Cervicogenic Headache. This is referred pain that comes from your neck but you feel it in your head often behind your ear. It is, however, often dull pain and you are describing more sharp pain. Cervical headaches are caused by irritated trigger points or hyper-irritable nodules or knots within a tight band of muscle — and reduced range of motion or a stiff neck.
Another possibility is mastoiditis. This is an infection of the mastoid bone which is directly behind your outer ear. However, you do not describe your brother has having had an ear infection prior to this which does usually proceed it.
Probably the most likely possibility is a pinched nerve. Was your brother injured at all? Gentle stretching and Nsaids often help tremendously.
Since you mention the left side, here is an article of pain and headaches on the left side. https://www.medicalnewstoday.com/articles/319406.php
The best thing for him to do is to be examined by a doctor in person. Please let us know what the diagnosis is. Best.
Hello~I agree with Emily's reply, I would certainly see the doctor as soon as possible to rule out any serious causes.
Please let us know what his diagnosis is.