The problem I ran into with the hay is that the oldsters couldn't chew it properly, so they couldn't get the nutrition from the hay. So even though I gave them lots of hay they couldn't carry the weight because they weren't getting what they needed from it. I'd feed as much senior and ultium as needed then, and soak it down a bit so it's a thick stew consistency, and let him work his way through it. The Senior and the Ultium all have pre & pro biotics in them, so I wouldn't think you'd need more. I'd worry less about feeding him hay and more about feeding him a complete pellet that can be soaked since he can't chew real well.
Since you're in CA you can probably get Sacate Mills Standard Bread and Thorough Bread pellets. They seem to be the best ones I've ever used and take soaking well. I haven't been able to get them east of AZ, but that's what I fed a lot of when I lived in CA & AZ.