How can I monitor an employee's activity on a remote Mac? I have a remote worker for my project and I am letting him use the Mac with the source code on it. I want to monitor his every activity, if that is possible.
If you do not trust the remote worker, do not hire them.

Legal, not technical

This is a legal problem, rather than a technical one. You should agree to a contract with the remote worker stating the limits you desire and how they will be enforced.

See Stopping certain data from ever leaving your Mac? for a related question.