If the check was deposited into an individual account in his name (not an exempt trust like an IRA, Section 529 account, etc.) then it is his money, free and clear, and is indistinguishable for tax purposes from any other money he may have.

Any tax consequences that may or may not have been due the grandfather for giving his grandson that money were incurred when the check went into an ordinary account, and in that year.

Any plausible tax issue would derive from whether he spent that amount on education in that year. It's moot now.