Unless he can get you a dump of the databases, you're being dicked around.
I work a fair bit with Plesk (unfortunately), he should be able to give you an individual login.
If you have access to WHM on the other end, you might be able to do a "Copy Account from Another Server", but again, you need the login details for just that Plesk account.
Let me know if you get stuck.
-Nicko