We had a pony boarding with us that has HORRIBLE dry skin - poor guy was so itchy he'd rub himself raw sometimes. After a skin scraping to eliminate the possibility of skin issues or mites (he was fine) our vet recommended bathing him with a 50-50 mix of Head & Shoulders shampoo and betadine surgical scrub a few times a week for a month. Now he just gets a bath once a month or as needed and it's done WONDERS for his skin. This mix is also super awesome to tackle mud scratches and fungus issues.

You can top dress his food with oil (corn, flax, etc.) or they make a lot of supplements for skin and coat that work pretty well. I saw someone recommend fish oil but I am not sure your average horse would find that very palatable. Also there is this stuff from Shapley's called Original MTG. It STINKS, it's oily, and it's nasty but it will solve just about any type of skin issue your horse has and it is not very expensive at all. That stuff is like Miracle Grow for manes, tails, skin issues, etc. My horse (Lonesome) came to me with fungus all over his neck and withers (he was neglected, poor guy) and this stuff got rid of it with two applications. I rubbed it all over him once a week and at the start of week three, it was gone. I washed him really well with an anti-fungal shampoo that has tea-tree oil in it and haven't seen a trace since. Another thing that is AWESOME is Amplify by Purina. It's a feed supplement and it's really made for weight gain but it does wonders for manes and tails, coats, etc. I sound like a salesman! LOL! Anyway, good luck with your itchy horse.

You can top dress corn/flax oil in small amounts - 1/4 to 1/2 cup per feeding. You don't need a lot. It will usually take about 30 days to notice a difference. Any form of calories will give your horse more energy but in small quantities, shouldn't effect him too much. A half flake of alfalfa will put way more steam in his engine than 1/2 cup flax or corn oil! If you are worried about O3/O6 ratios, you can always use canola or soybean oil. Their 3/6 ratios are better than the corn oil.
I am not sure what you are feeding him but you can also look for a comparable feed with a higher fat content. Say you are feeding a 12% protein, 4% fat, you can always switch him to a 12/8. That goes for pellets as well as sweet feed. That way you don't have to worry about measuring out the added fat with oils and such. I have found that regardless of age or breed, the lower protein (not less than 12), higher fat feeds make my horses look better overall and they don't get as hot as when they are on higher protein feeds. Those are my horses though and what is good for them may not be good for others.