Dad had non-hodgkin lymphoma & died, what are chances that son will get it?

Very low...: Although large studies of populations do show a statistically increased risk for family members to get a non-hodgkins lymphoma, this is still an uncommon cancer. So the son's chances are still low, although a bit higher than the risk carried by the general population.

Unlikely: Any family history of cancer, especially in first degree relatives, may slightly increase the risk, but the chances are still very small.