K.,
I liked all of the responses on how to give your husband and son time together.
My suggestion would be to put him into a church daycare or something similar once a week for socialization. You said you are concerned he is not playing with other children, the daycare would give him an opportunity to play in a safe environment without you there.
If you do decide to go this route take him to the daycare a few times and spend some time playing with the toys, meeting the adults and other kids. Show him where he will be napping, eating snacks and lunch, and all of the fun places. This way when the big day arrives he will already be familiar with the daycare and it won't be so scary.
You might want to take pictures of the daycare and have them printed onto a few sheets of paper. Then the night before the big day bring out the pictures and talk about the fun activities and new toys he can play with and the new friends he will make.
Plan for crying and temper tantrums the first few times you take him, but eventually he will remember the fun he has and won't be so sad. Just be firm, give him a big hug and kiss when you drop him off and then leave on a happy note. The adults at the daycare know how to help a child who is crying and how to engage them in activities.
I think it is great you want your husband and son to have more time together and your son to have more interactions with other children.
Good luck with whatever you decide!
-C..