I suggest you tell him to make it plain and obvious, in his postings and comments, that YOU made the videos and that Universal owns The Land Before Time. If he's not mentioning you or trying to make it look like he made them, then you should get him to make it clear that you made them. If he doesn't comply, send a complain to YouTube (though, regrettably, I see a lot of plagiarism like this on YouTube all the time, plus, I doubt MVs are even "legal" on YouTube...).