How will my husband's future fertility be affected after the loss of a testicle?

No. Men with one testicle should make a normal amount of sperm as long as their pituitary gland functions normally.
Hopefully not. Most men have a huge reserve of sperm, and a single normal testicle produces more than enough sperm to remain fertile. If husband needs to have testicle removed for cancer, fertility may depend upon any subsequent treatment required. This should be discussed with his urologist. Sperm banking may be advisable if he requires certain types of chemotherapy or abdominal lymph node dissection.