Terminating an incompetent, disrespectful worker

We promoted an “adequate” 50-year-old salesman who had been with us for nine years so we could justify his monthly salary. A few years ago, we gave him additional responsibilities and an increase in salary as a result. He is still doing an adequate job at sales and the new position, but is constantly complaining about the new responsibilities.
By Tim Mitchell
|Canadian Employment Law Today

Question:

We promoted an “adequate” 50-year-old salesman who had been with us for nine years so we could justify his monthly salary. A few years ago, we gave him additional responsibilities and an increase in salary as a result. He is still doing an adequate job at sales and the new position, but is constantly complaining about the new responsibilities.

He has been disrespectful towards managers and supervisors, and has been blatantly disregarding his hours of work. He is also spending three hours a day surfing the Internet for personal use. We are hoping to sit down with him and set out some improvement strategies. If we give him a set amount of time (say three months) to improve, and he doesn’t, can we fire him for just cause? Failing that, can we let him go without cause? If we let him go without cause, how much severance are we required to give him?