In any case, the suggestions seemed to fall into one of three categories: food, health, or training. I agree.
Food: after talking to my vet and the newest trainer, putting weight on him is important right now and my feed regimen, minus any grain, seems a good choice for him. He is receiving one and half large flakes of alfalfa twice daily. I call it free choice because he never actually consumes all of it, which is by design. Speedy is fed this way also. I really like my horses to nibble all day. Eating combats boredom and keeps the gut moving correctly. He also gets about three pounds of soaked beet pulp with an added two pounds of rice bran (fat source). One of my favorite vets and equine nutritionists, Dr. Susan Garlinghouse DVM, MS, has an online article that provides details about the "energy" of most of the feeds we give our horses. You can find the article here.
Compare the energy content of beet pulp with other grain and forage sources:
Feed Type - Energy (Mcals/kg) - Comparison to beet pulp
Vegetable oil 8.98 - 385%
Corn grain 3.38 - 145%
Wheat bran 2.94 - 126%
Oat grain 2.85 - 122%
Beet pulp, dry 2.33 - 100%
Alfalfa hay, early bloom 2.24 - 96%
Alfalfa hay, full bloom 1.97 - 85%
Bermuda hay, 29-43 days growth 1.96 - 84%
Timothy hay, mid bloom 1.77 - 76%
Oat hay 1.75 - 75%
Orchard grass hay, late bloom 1.72 - 74%
"Notice that although beet pulp is higher in calories than any of the forages, it is lower in energy than any of the cereal grains commonly fed to horses... While beet pulp is lower in energy pound for pound than grain, it is also lower on the glycemic index than any of the cereal grains. The glycemic index is a comparative indication of the simple sugar content of a food source, and of its relative effect on plasma glucose. Feeds with a high glycemic index, such as corn (which is high in starch), break down enzymatically to glucose very rapidly in the small intestine, quickly elevate the blood glucose levels, and in some horses, may contribute to "hot" behavior that make early-morning, high-octane starts about as much fun as riding the Space Shuttle bareback."
Health: Sydney saw Dr. Blanton today, Tuesday, for a physical exam and got an a-okay. His teeth were fine, if not overly floated. We tested his stool for worms and blood, in case of an ulcer, and got negatives for both. She listened to his heart and gut sounds and found all was well. She cleaned the wax out his ears, but found nothing alarming. She also examined his eyes which were fine. She found some scarring on his hind legs, which is easy to see, but found it to be well healed with no cause for concern. Overall, she liked his mellow attitude and thought that he would make a fine dressage horse. As a side note, Dr. Tolley also liked him and gave a positive nod to Sydney's new career path.
Training: Sydney and I have had four lessons, all of which have gone very well, especially the one we had yesterday. Both Coach and the newest trainer have given him positive comments and feel that he and I should progress quickly. He has been turned out several times since Saturday with none of the explosive behavior that I saw on Saturday. He seems far more relaxed the last three days and was a rock star at the the vet hospital. (Dr. B and I left him in a stall at the hospital while we went for lunch. When we came back, most of the hay we'd left was eaten and he was hanging out with a very relaxed attitude! - So much for Mr. Hyde.)
In summary, I really think that eliminating grain and giving Sydney some more time to acclimate to his new home should eliminate all but the most infrequent appearances of the dastardly Mr. Hyde. Thanks to those who offered suggestions, and I'll keep you posted.