To manipulate his service time to get that extra year, they have to keep him under 172 days in the Majors for this year and next year, so 171 like the Cubbies did with Bryant.
If they call him up September 1st and keep him til season's end, they'd burn 30 of those 171 days. That means they'd have to wait until end of May to call him up as opposed to end of April to begin next season to keep him under 172 days.
It's a shitty and unethical thing that teams can exploit but having him for that extra year could be crucial in the Jays possibly winning a world series even though it robs fans of seeing a young star who's ready to perform in the major leauges.