It's very likely that an attempt on his part to sue Microsoft would fail on the grounds that a tacit agreement was reached for the use of the work, based on his

It's very likely that an attempt on his part to sue Microsoft would fail on the grounds that a tacit agreement was reached for the use of the work, based on his acceptance of payment. However, it doesn't sound like there was any agreement, tacit or otherwise, that the license should be exclusive. So he's still well within his rights to dedicate it to the public domain.