Assuming your son does not have a terminal illness or may die sooner than what's normally accepted.
I usually say that you will likely live beyond 100 years!
A 100years is a LONG time. Even as your son's immediate parent you may not even be half that already yourself.
With the modern advances in cancer research and people eating much healthier foods, plus with physical sport and generally living a good healthy normal life, your son could easily live past 100 as long as he takes care of himself.
In the 'old days'. It was cool to smoke (sadly I still smoke, but seriously contemplating giving it up this time!) NO one jogged or ran in the mornings, I mean, why would you? And food usually consisted of hamburgers and stacks of butter on sandwiches and biscuits. Eating your 'greens' was frowned upon by most children.
These days, children thrive on yoghurt. They probably don't eat butter at all, and possibly never have. Meat is not the number one (feed your man meat) meal. Most responsible parents usually prepare all types of different healthy meals each day of the week! And which child isn't into some type of sport (at 8) these days?
Yes, children today are set up for a long and healthy life.
100 years to any child (and even adults at 50) is a LONG time.
In a hundred years there will likely be even more advances. Who knows, he could live to 150 or more!
Your son could be a great great great great great grandfather before his time is up, and his wife may not even be born yet!
What to tell your son?
Don't work matey, you're 8. I'd like to know what its like living on Mars or having trips to the moon. In a hundred years they may even have a cure for death. ie You've seen those heads in Futurama? Well they're 1000 years old, yes its a cartoon, but who knows what the future will bring.