I feel for you - our 5 month old has pretty bad eczema. We've even had to resort to a medicated cream to calm it down for a while.
Has your doctor said what the rash is? It kind of sounds like eczema to me.
There are a lot of ideas out there about baths and creams, but we've done a LOT of research and trying things out, and found that 2 baths per day work really well for our little guy. For his head, you'd have to keep wetting it down (cuz you're not going to dunk him for the whole bath!
You can let some conditioner soak on his head/hair for while, and then rinse. And afterwards, brush his head and get some of the flakes off.
We've used California Baby Calendula Cream and that's pretty soothing.
The thing about eczema is that the scratching just makes it worse. Which sucks, because we're constantly telling Z. "don't rub" and he sure doesn't understand, and gets pissed when we hold his hands away from his face. But if he rubs, it will itch more and cause more eczema and flaking and thickening of the skin.
If your son doesn't have hair, you can apply a good moisturizer directly to his scalp multiple times a day. Aveeno Baby is really good. Eucerin is okay, but you need to apply it to wet skin, so it can lock in the moisture. So you could lay a wet washcloth on his scalp for a minute and then put Eucerin.
If your doctor doesn't have any idea what it really is, and is just randomly suggesting dandruff shampoo, then get a referral to a pediatric dermatologist.