Do you make him transition fast? That can be really hard on people.

"You'll need to change activities in 5 minutes" or in a game "One more turn then we're done"

Even maybe with a visual clock for how much time is left. The visual analog timer clock things are cool.

Suddenly asking for a transition is very sudden.

The other way to approach it is to transition to something short and nicer, and then down to what you're wanting to do. Thing that is liked -> Thing that is liked more -> Thing that is liked less
Where thing in the middle is shorter and less getting stuck in it mentally, but the increased preference makes the transition to it from the first easier.

Similarly, stepping down, and organizing what order things are in so it goes
Special Thing! -> Fun predictable thing -> Thing that's not as nice
makes transitions easier than going Special Thing -> Thing that's not as nice
(Like in a school: Party -> Recess -> Schoolwork instead of Party -> Schoolwork)