I have one like that - nearly 8 years old but as fit and manic as she was as a pup - fortunately for me she is only 12 inches high but I can understand what you are saying :-)
I would think the first thing to look at is diet - some dogs are incredibly sensitive to colouring and flavouring and need a very natural diet. If you are feeding something high in additives you could find huge benefits in changing to a simpler food. Remember cost does not always equal quality. I'm in the UK so won't bother recommending foods but I'm sure others can.
Have you tried to find some other ways to stimulate his mind? I don't know if you have Nina Ottosson toys in the States? But they are highly recommended for active, intelligent dogs who need to use their brains to tire themselves out.
Nina Ottosson Interactive Toys