Are there any supplements/treatment for older dogs with good motility but low fertility?

Fit and Fertile do a supplement but can't remember the name,if you look on their site you will be able to find it.

http://www.fitandfertile.com/canine-fertility/details/61/8/canine-fertility-products/target-1kg-for-stud-dogs?keyword=target

It's called Target

Basic joint aids also contain selenium omega and sperm boosting properties. I'll Googlee for the link

http://www.poochandmutt.com/breeders/fertility-supplement-for-male-dogs/

Has the results been seen in litters or just under the microscope?

Under the microscope. 

So you mean good motility low frequency? Low.sperm numbers? Its only fertility testing.when fertilizing?? Wouldn't worry to much unless he's missing bitches regularly or very small litters. Although I believe its down.to.stud management mainly.