Just a thought...
Doing math problems alone and feeling satisfied and happy about it doesn't have to be a sad future. Some people love math. Some people are introverted. There is nothing inherently bad about being a person who gets joy in this way.
Of course you want your kid to be able to connect with people in ways that satisfy him. But, I would be open to the idea that while his way may be very different from what you would enjoy or from what you imagined that doesn't mean it is sad. In fact there are quite a lot of happy actuaries, mathematicians, etc. in the universe. There is every reason to believe that a person with these interests can find a happy and satisfying life. If anything I'm less worried about kids who have found joy through solitary intellectual pursuit because there are so many ways for that to work out in a happy adult life.
My suggestion for now is to seek out opportunities for engagement that are not just with kids his own age. Ideas: chess club, computer users club, getting to know elderly neighbors or friends who might serve as mentors, community service, game groups, etc.
