Too many unknown variables...
If he has any high interest debt pay that off.
If his only debt is very low interest rates, keep the debt and pay over time.. I know some folks with like 1 or 2 % car loans, might as well keep those.
If he doesn't have a financial adviser he may want to look into getting one.
Has he looked into purchasing any kind of long term care insurance in the event he needs care when he gets older, without being a burden on his children?
He probably should look into a financial adviser to help him make decisions...
I personally would suggest mutual funds over individual stocks, if he wishes to invest. He may also want to do dollar cost averaging. So maybe he decides to invest 1K per month over 4 years time period. So if the market goes down a bit he can buy more shares at the lower price. Prior to investing all the money into mutual funds the balance can be rolled over into some short term CD's.
With the market at it's peak in many years, it's possible it may go down further. Of course, it could easily keep going up to..
Some things I would not do...
If he has grandkids, I would NOT put it into their name for a college fund (529 type). The reason is colleges look at that money and reduce amount of aid the student gets.
I'm all for helping out the kids\grandkids, but if it's in his name, if he needs it later, it's still his. If he doesn't need it when they go to college, it's not in their name and he can give them money as they need it without it really impacting their grants\aids for college. So hopefully they can graduate with minimal college debt.
Sorry, for not having one good answer, but I don't think there is one good answer.