I don't know anything about a 90 day law, not in NJ anyway.
Businesses often have a probationary period when they can get rid of you for no reason. Truth is, they can get rid of you at any time for any reason (unless you have a contract that states otherwise).
I always thought it should be called a 90 day trial period. The person's not on probation, just being tried out for the job.
And unless you have a union or other contract that restricts this, keep an employee as long as he's good for your bottom line (and mental health). As soon as he isn't, get rid of him.