That's right around the age when children should start to be able to handle sequenced events (mine is 3.5 years old and definitely working through it.) Say something like "first, we'll read a book. Next, you can play a game on the phone. Then, it's time to have dinner." Whenever you transition, remind the kid of the sequence. He'll start to get comfortable with the routine of doing something for a while and then moving on to a different task.

Some kids respond really well to a "visual schedule", where you have pictures for each event with velcro or magnets on the back, and a board to connect them to.

It can and will be a battle, simply because it's hard for a little brain to develop things like impulse control, delayed gratification, and patience. But it's important for your son to learn it, and this is a great age to work on it.