Assuming that all of your high denomination chips were clearly visible (pretty safe assumption I'd say), then he doesn't really have much reason to back out of the call. Based on the information available it seems a pretty cut and dried case of verbal being binding.
This is exactly what I said, but for some reason this guy for most of the game was just kind of looking down or looking away from the table, not really paying attention to anything like how many chips someone had. Not making an excuse for him but I didn't want to argue to the point of getting another player mad. He is a newer player and it's the first time i've played with him. He was given a warning and told that if he did that again he would be forced to call no matter what the bet was if he said call.