regardless of what causes it, you must make him learn this important skill. Being able to play on your own as a young child translates to being more able to do your school work on time and on your own, being able to make friends (I know that sounds counter productive but it's true) and being able to be a better functioning adult. Remember our goal as parents is for the kids to function without our help and eventually leave us.
Now how to fix it. Start out small. Say you and he are playing legos/duplos. Show him how to build something new and then tell him you need to fold laundry, cook lunch, go potty, whatever and that you'll be back in a few minutes to see his building. Then go away for about 3 minutes, come back and congratulate his efforts if he did them. If he didn't do anything tell him to call you when he has built it cause you need to get whatever done.
Keep doing this with different toys you own. Teach him how to play with each and every one of his toys in this way.
Depending on his age you may also want to enroll him in preschool as this is a major skill I teach in my preschool and most preschools should teach that skill.