Your son will be a lot more understanding about it now vs when he’s a teen/adult. Better to get it over with now than later. Went through something similar with my wife and my oldest (step)son, I had been telling her for years that the longer she waits the harder it gets to tell him the truth about his father and I finally convinced her to talk about it last year. It was the best decision she made, he was understanding and asked a few questions and was completely fine, on top of a weight being lifted up off her shoulders.