OP- hugs.
Two thoughts- you can honor your mom and her intentions towards your son by continuing to express your pride in him, and your belief in the value of his education. I feel from your post that her contribution was both practical and symbolic- and the symbolism is by no means negated by the fact that she didn’t live to see the fruits of her labor. I’m sure your kids were her pride and joy…
Second- do you know for a fact that ownership of the 529 passed to your dad when your mom died??? Depending on how the account was set up- your mom could either designate a contingent owner (which logically would be either your dad or you) OR the account would/could revert to the beneficiary, I.e. your kid.
Do not assume. It’s worth checking…it would be in the original paperwork, or a quick call to the financial company where the account is held would likely yield the answer. People don’t always retain the original forms, but the brokerage firm where the account is would have a record of either a contingent owner, or what the particular plan’s rules are for when a 529 account owner passes before the money is used…