I have very severe itching and flaking of the scalp. I was told by a dermatologist that I "might" have sebborheic dermatitis. What shampoos can I use to help with the problem of stubborn itching? I've tried head and shoulders and demorex.

Seb derm. I'd use a tar based shampoo, zinc based, or a salicylic acid based shampoo. Sometimes its good to rotate them, using one one day and another the next. Leave in 3 min before rinsing. I would not advise a tea tree shampoo as these products can cause gynecomastia in men.
Shampoo. Two good ones are selenium sulfide 2.5% (selsun) and tea tree oil shampoo (5%). Both are effective and well tolerated. Prescription medications may be needed though.