Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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All the best,
Dr. Getty
Hello Susan,
Thank you for your patience in my getting to your question. 
I'm assuming that by white spots, you are referring to the hair coat, and not the skin. Changes in haircost coloration can be favorable or unfavorable, depending on the situation. If a horse has been undernourished and is placed on a nutritious diet, the hair coat will show its true coloration and variations in color. So, if you've improved your horse's diet over the past two years, you will likely start to see his true colors expressing themselves.
On the flip side of this, of course, is the lack of appropriate nutrients. When faced with a nutrient deficiency, the horse's body will use these nutrients in order of priority. His first priority is to stay alive, and will therefore, use what he has available to protect his vital organs (e.g., heart, liver, lungs, kidneys, etc.). As a consequence, his outer tissues (e.g., skin, hair, hooves) will suffer. So, a horse that looks good on the outside is generally healthy on the inside.
Since you haven't mentioned what you're feeding, my first impression is that he is lacking in enough copper. Copper is essential for proper hair coat coloration. And, this is easy to fix by adding a vitamin/mineral supplement to fill in the gaps in his diet. If you are not already feeding such a supplement, consider adding Glanzen or even Glanzen Complete to his diet. Give it time -- at least a year -- but you will notice an improvement in his overall health.
And, please do keep me posted!
All the best,
Dr. Getty
Where are you from? Bayfield, CO