I am sorry that your husband has Varicocele
with nil sperm count
As your doctor said,it is 50-50 chance of improving sperm count after surgery.
You can go for Testis biopsy and confirm the correct pathology.
You have to consult an Infertility
and proceed accordingly.
He can take treatment and improve his sperm count,but with the advice of Urologist/Specialist only
Wish you good health and all the best