Etiquette Question

We have this friend, D, who has been friends with S since they were in high school. D is an odd mix of hyper-sensitive about protocol and yet clueless about a wide array of social norms. His wedding, for example, was the most intricately formal affair that I have ever been to, carefully choreographed (by him, not his wife) to keep Emily Post perfectly happy. He is still humbly apologizing for something that he did to me, by accident, more than eight years ago, something that I laughed about at the time and was never upset about, and for which I keep kindly telling him that he really doesn't need to be concerned anymore. He has spent holidays with S's extended family, and interrogates us carefully about the preferences of various members of the family, so that he can be careful not to offend.

And yet this same guy regularly invites himself over to our house for dinner, whenever his wife is out of town, by explaining to us that he would like me to cook dinner for him because he cannot fend for himself. And he is constantly explaining to us how brilliant his son his, especially compared to most normal children (like LL). He even makes his son put on little "shows" for us, to display how much he knows. It is ... weird.

We recently went to a birthday party for D's son, who turned four. We bought him a small gift (Lego cars!), LL happily handed it to him, and he immediately ripped it open. And then declared loudly that he didn't like it, before tossing it aside and running off. D was absolutely mortified. He kept apologizing for his son's behavior, throughout the evening. Bizarrely, he never actually corrected his son, or encouraged his son to behave differently; he just kept apologizing to us.

But, whatever. The kid is four. I wouldn't be shocked to see any four-year-old behave like that on occasion, I've seen much worse from this one, and I've seen similar behavior from D, so I wasn't exactly surprised, and I didn't particularly care.

Fast-forward to today. We received, right on time, a nice thank-you note from D's son, hand-written by D. Here is the card, verbatim:

"Thank you very much for the Legos. While I rudely said, "I don't like it," my attitude changed dramatically when it was exchanged for nerf guns."

So... I laughed out loud. Because it is so typical of D -- he did the appropriate thing and sent a note, but the note itself expresses what a horrible gift it was, until it was exchanged for something much better. And on the one hand, I guess I'm glad that they were able to get the kid something that he wanted (that is, after all, why I carefully included a gift receipt in the birthday card). On the other hand... is it normal to tell a gift-giver that you exchanged their gift? I would think not. And certainly not in a way that makes it obvious that you hated the original gift. (Surely there are more tactful ways of mentioning it, if you feel the need.)

But now I'm actually torn about what you should say in a thank-you note when you didn't actually want the gift. I mean, that note is obviously not it, but what is the right thing to do?