Is it really seborrheic dermatitis?

My 3 yr old son has been suffering from a scalp problem for about a year now. The dermatologist say it is Seborrheic dermatitis. However all the information that I have read does not necessarily describe his problems.  He gets bumps on his scalp sometimes just a few and some times a lot. The bumps are painful to touch and look like pimples under the skin or pimples with pus in it.   Now the derm has placed him on different medications plus a otc shampoo such as t-gel.  However the problems still presist and is rarley under control.  My son is african american and the doctor told me to reframe from putting any type of oil in his hair.  This is a problem in itself plus he gets his hair cut very low and it seems to only make the problem more prominent.  Also he has lost hair in a few of the places that the bumps were after they heal. Is this truly symptoms of seborrheic dermatitis?  I feel so lost.