Sometimes, even with good nutrition and a good trim, building adequate sole is a little hard, esp. In older horses who have slower growth in general.
Assuming your trimmer is doing a fabulous job and there's not any infection and your horse is getting the best nutrition for him, age and lack of exercise are probably your more pressing factors.
Even "lots" of exercise by your standards probably isn't as much as you think. I'm glad to read you are using hoof boots, too!
Try using some pad inserts, if you aren't already. You can try leaving the boots on half the day with the pads. I find pads can build sole in a hurry, but a hoof needs exposure to air, and they sweat in the boots and get over moist.
Another alternative is to ask your trimmer about a cast that can hold a foam pad on the hoof for up to 4 weeks at a time, or alone. It will breathe better than a boot and can really help laminitic hooves. This is a tool I've recently discovered and am LOVING it.
Finally, you could even add pea gravel in a high traffic area. Up to 3 inches thick. It's supportive, yet stimulating and foundered horses love it. It does exfoliate at the same time, so instead of crumbly sole, you get smooth, hard sole.
Then, regardless of what you choose to stimulate the feet, spread his hay out in lots of little piles, make him work for his food. Each step he takes to "graze" will help stimulate good sole growth. Remember, hoof growth is measured in miles, not in months. The more he moves, the faster he can build healthy hooves.