When an employee gives a 2 week notice, can the employer release him right away, without paying the two weeks, and not paying for workman's comp?

The very nature of at-will employment is that the employer is free to terminate any employee at any time for any reason or no reason at all. You have no more duty to retain him for the next two weeks than you would have if he had not given notice. Your questions about workers comp do not really make any sense. All employees are covered by workers comp, whether they are at-will employees or have contracts with a longer "duration" term. If he is injured on the job prior to termination, termination does not cut off his right to collect comp for injuries suffered while employed.