To get a caveat on a property, you need to have what's called a "caveatable interest".
You might (very, very, very might) be able to get one
on the basis that he owes you an unsecured $X-thousand dollars.
But that's only for the money you lent him - not for the proceeds of any sale, and
not because you're afraid that he'll blow it.
And, you'll need the help of a lawyer to do it.
A mere fear on your part that your (presumably adult and legally capable) son
might be irresponsible with money in future is not likely to give rise to a caveatable interest.
Sadly, the law does not protect otherwise able people from making foolish decisions.
It only provides - sometimes - a way to clean up the mess that a fool makes.