Obviously, he can get all the data stored in the luks header. That's basically the cipher used for encryption, the salt, the number of keys that can be used to unlock the data and so on.
If you've filled the USB drive with random data prior to encrypting it, that's basically the extent of the information anyone can get.
If you haven't overwritten the USB drive with random data, an attacker will likely be able to determine how much data is stored on the partition, and making assumptions about the underlying filesystem, he could gain insights into the structure of your filesystem (e.g. how many files are stored in how many directories etc).
AFAIK, the date of first encryption or last write can't be determined from the LUKS header.
For someone with very good hardware knowledge, it might be possible to access the wear-leveling protocols on the drive and therefore gain access to previously written data, which, even though encrypted, would give hints about which parts of encrypted partition were changed. I'm not sure what anyone could do with that knowledge.