My stab in the dark at it is this:
He probably wouldn't have appreciated Hollywood making The Hobbit it's own, with so much new having been invented to stretch and flesh it out for three movies. His book becomes barely an outline and he might've taken exception to that, at least until the check cleared.
He would've noted that The Fellowship of the Ring was actually good, certainly compared to its sequels. Many of the changes to the story can at least justify themselves, it's often visually impressive and is very charming. There's not much to be embarrassed by, here, despite Hollywood's jazzing it up, here and there. It's not half bad, as a poster boy and he might've even felt proud of it.
"In movie form, LOTR had a strong start, but a weak finish," he might even say - and with the increasingly unnecessary liberties taken for the sequels, they may not have found favor in his sight, hardly at all. But with all of the crazy money he would've made, certainly when the movies were hot, I, personally, wouldn't have been especially interested to hear any of his bellyaching. If you sold the rights, you sold it.