I also strongly recommend that you contact the EArly Intervention people. It is true that some services are limited after the age of 3 but my 3 yo grandson is in preschool and getting speech therapy through that office.
They do a thorough assessment to determine what if any services he needs. They can also rule out the serious issues including autism and aspbergers. Wouldn't it be a relief if they confirmed your idea that he only needs socialization. Providing for socialization is part of their program. They provide services so that children can be successful in school.
I don't know why you had to seek a different route with your daughter but don't let that stop you from getting help for your son. He is a different child with different needs.
And sometimes it is difficult dealing with the system. But it is better to deal with the system now instead of later when he's in school and still having difficulties. You still have a chance of correcting some difficulties by age 3 but doing so gets more difficult as the child gets older. They told us that it would've been much better for my grandson if we'd gone to them several months earlier.
Arranging for your son to be around other children is the best way to try to get him involved with them. You could try doing some of the things that are recommended for doing with younger children (around 2). Play with him and another child. Show him by example how to play.
Does he play with his sister? If he doesn't I'd really urge you to get him in for an evaluation. It really is easier to deal with difficulties when they are younger. I'm not saying it's easy. But it gets more difficult the older the child gets.
The internet does have some good sites to learn about Early Intervention services. And you can call their office.