Is a single sperm capable of being made? Probably. Can a Single sperm fertilize an egg? Certainly.
The issue is, guys with low sperm counts have issues all of the time getting a woman pregnant...I would imagine, that if any sperm production is to be had it would be incredibly small. So very unlikely to be useable. Possible I would imagine, but unlikely.
I sympathize. I lost my mother this year too she was 53. We weren't terribly close, but the thought did run through my mind of a legacy, but I decided that other siblings would be plenty fine seeing as they reproduced in large numbers already, and I am not sure I want children anyway, sometimes I really do, then other times I really don't.