Plan

They're all okay. I suggest: His plan to allow his son to go to college failed.

I'm not sure if you mean that he had a plan that would allow his son to go to college (that is, his plan was "I will do [something] and that will let me earn enough money to pay for collge for my son") or if you mean that his plan was "My son will go to college."

Your orginals and Philip's suggestion are suggest the first meaning.
If you mean the second, then "He planned on his son's

Thank you very much Philip and Barbara.

Hello Barbara,

I think

A-His plan was for his son to go to college.
B-His plan was that his son would go to college.

would be ambiguous and could have both of your meanings.

Am I correct?

Gratef

Both of those suggest to me that the plan is "My son will go to college."