I think must be investigated for FSH LH Prol estradiol ,varecocele .

This is a simple and small question to a very large and complicated subject....infertility books are better .

Reference to the specialist.......

Remove the causes, if you find it.... look for infections, or drug intake, otherwise you can only counsel well the man and the couple and avoid the supplements (aminoacid, vitiamins, antioxidants, etc). this is my point of view.

Treat the cause like varicocele or infection . L-carnitine, acetyl L carnitine & antioxidant can improve motility.