Your son is probably worried about his dad's business planes crashing. He's indirectly asking you for reassurance that they won't.
I would not give him a long lecture on the safety of airplane travel, or lie to him if the planes your husband flies are indeed similar in size to this particular plane. Instead, acknowledge the aircrafts' similar sizes, but explain that his daddy only flies on planes that have very experienced pilots in command, which is not the case in this unfortunate circumstance. Moreover, I would explain to him that the pilots who fly his daddy would not have flown their planes at night under these conditions. And that his daddy only travels in small planes and big planes when "everything is safe and right" for flying.
I would be mindful of keeping him away from the media frenzy accompanying this tragedy, as it will certainly include frequent speculation about pilot error, the unsettling number of small plane accidents and near misses, etc. He may become more fearful and anxious as your husband's next airplane flight approaches. It may be helpful for your husband to tell him about how great it has felt to fly on his small business planes. He can mention how he has traveled so many times like this and always been fine. The "always" part is what your son needs and wants to hear at this time. It might be OK to have him draw some pictures of his daddy flying on a plane, waving to him from the plane's window as the plane lands safely. Feel out how much reassurance your son needs by asking him a few open-ended questions, "Why do you think they're showing that plane on TV? What do you think about Daddy flying on planes that size? Would you like to fly on a plane like that?"
Your initial short response and reassurance may be all he needs but he also may need periodic reassurances during the next few weeks and when your husband flies next. Let your son give you the behavioral clues as to how much more he needs from you on this issue. Not talking about it at all would be the most damaging response.
It's not just kids who worry about their parents flying; parents worry a lot when kids have to fly alone. For more on kids and flying alone, ckick here.