This works every single time!
Incorporate him in your activities.
He will do 2 things. He will be a helper and he will eventually get bored with the activity and learn to entertain himself.
So when you are cooking..give him some roll cookie dough to 'cut out' cookies, a bowl of milk and cheese from box macaroni to 'mix and stir' for you, or even a pitcher of koolaid to stir up. Then he can have some of his treat he made. He can also learn to do other things like mash the potatoes, rinse the lettuce. The jobs will either keep him entertained or lead him to decide on his own he would rather play with his toys than do that job. ;-)
He can pull up sheets and blankets to make a bed, sort socks, sort dirty laundry for wash day, wipe down the counters with a damp rag. All this is also training for someday when he needs to clean up his own place. ;-) Even a 4 yr old can do all these things!
You don't give him a choice. You say this is the activity right now. Help or go play. If he helps, great. If he doesn't no loss, he has made a decision to play by himself. LOL
Mine helped me do EVERYTHING on some days. But I considered it our time together. Other days they decided it was no fun and chose to go play with their toys.