Ummmm...could be used to watching TV and just sitting there with 3 older siblings and some kids are quiet. Having said that ~ he might be showing signs of Autism. I am no expert but I have a friend who has a son with Autism and she mentioned to me that their signs were that he didn't play 'normally' with toys and other children and would just sit there often. Not making eye contact is another thing he did.
I'm guessing that you've tried to get him to engage in activities ~ crafts etc. He doesn't want to?
Nothing I would tell the parents he might have but just one of the first things that popped into my mind with a friend who deals with a little one who won't play either.