If your answer is 'YES' to my question above, then I can recall that system DSN can be used by everyone who is a user of that system. When you make this guy a server admin, the DSN is accessible to him otherwise not. Try adding him as a normal user(not any admin) to the server and then verify if this guy is able to use the DSN. also, we need to make sure that remote desktop ability for him remains disabled so ask your windows guys to assign him the least permissions.

Try this please.