Under Sec. 79.4 of the Quebec Labour Standards Act, the salary increase would have to cover the full year, including the 4 months he was away on leave. Here’s the language (I boldfaced the key part)
79.4. At the end of the period of absence, the employer shall reinstate the employee in the employee’s former position with the same benefits, including the wages to which the employee would have been entitled had the employee remained at work. If the position held by the employee no longer exists when the employee returns to work, the employer shall recognize all the rights and privileges to which the employee would have been entitled if the employee had been at work at the time the position ceased to exist.
Hope that helps and sorry it took me a week to get back to you. Glenn