The fertility dr should be able to organise tests and advise if any supplements would help you. My husband has low sperm count and motility. He had chromosome blood tests to check if he was a carrier of cystic fibrosis, as apparently this gene is likely to cause issues with sperm. He wasn't a carrier as it turned out. He was also given a supplement called "Fertilix" which you can only buy online, but unless you have been advised be a professional I can't say if it made any difference.
Good luck in finding some answers.