Purina is down there for low quaity feeds - I dislike Purina greatly.
I had my fellow on Tripple Crown Senior, and after discussing that product with an Equine Nutritionist at the Michigan State University, I found out that Tripple Crown is up there in the totum pole of quality feeds.
If you want what's best for your fellow, I say full turn out as much as possible. Little stall time - have him in front of a round bale, where he can stuff his face 24/7, with pasture as well.
Him having access to as much Vitamin E as possible great.
I would also look into a digestive supplement, like SmartPaks SmartDigest Ultra. That stuff works great - many horses, especially senior horses no longer have all that is needed for their digestive systems to work proper and as efficiantly as a younger horses system would. With Senior horses, their systems have a harder time digesting and processing and using all the nutrients needed that they get from their feed - so by adding a Digestive Aid Supplement will help.
And of course, the feed. If you go with Tripple Crown Senior, you are getting a lot - Shaved Beat Pulp, Rice Bran and plenty of other important ingredients needed. I would feed 6lbs+ a day, and seperate it into 3 feedings.
When I was feeding it, my boy was getting 10lbs a day.
Here is an article that I find very helpful: Balancing Act - Designing A Diet For The Modern Horse