If the pastures are lush, then he really doesn't need any hay except when he's inside. To keep him from getting hyper, keep him turned out as much as possible, preferably 24/7 with just stall time to eat a little or during bad weather.
I would slowly switch him over to a ration balancer feed. Call your local feed stores and see if they carry one. Ask for a specific product name though, as most feed stores haven't caught on to whole ration balancer thing ;). Here's a good list of most of what's out there: Specific Ration Balancer Products
You'll want one formulated for grass hay. You can also Google each company and check to see who's a distributor in your area.
You feed just 1-2 lbs of a ration balancer daily. Slowly switch him over to it over a two week period. If he' holds his weight fine, you don't need to add anything else. If notice him dropping weight, add more hay. You can also add plain whole oats to the RB and/or a fat source like corn oil or flax seeds. Most horses keep their weight fine with just the RB though, as long as they're getting enough hay.
If your local feed stores don't have any, then I would do what I do, feed a simple diet that includes a vitamin supplement for horses not on any fortified grain. Smark Pak has a line called Smart Vite for this purpose. Otherse include Select II, Balance, II, Uckele's Equi-Base Grass, Linpro, Mega-Cell, Augment, etc. I feed one of these along with 1 lb of whole oats and 3lbs of chopped alfalfa hay (or alfalfa pellets). My hay is all grass hay. My hyper gelding has really calmed down and my mare's heats are much less noticable. They also have better hooves & hair, less rainrot, and less thrush than as compared to when I was feeding them a commercial feed.