How to politely ask him not to send me so many short messages?

Obviously, this man has no good intentions for you.

Don't be tactful, and don't be afraid of not giving him face.

Such a thing "cut the gordian knot"

If you want him to leave you alone in the future, just say so.

You can tell him, "Sorry! Maybe the age gap between us is a little big, we are not friends, or we forgot to make friends! I hope you can respect me and yourself!

This kind of message is euphemistic and not abusive!

Of course. This is just my little suggestion. As for how to do it, it depends on what you think! Wish you all the best!