This is actually one of those situations where instead of getting rid of children already in care you might consider adding another child to upset the balance or even out the balance depending on your point of view. There is a reason the saying "three is a crowd" came about. The boys are used to each other and don't see any reason to include the younger child. It would be the same whether the younger one was yours or not so keep the whole situation in perspective. You stayed home to be with your child so this actually works well in that you can play with him and the other two can play together and you get paid to play with your son. I know that isn't the whole plan of course. With time they will all start to play with each other. Your son will grow into the stage of play the boys are at and it will be easier for him.

For now you may want to plan some group activities so they can learn to work together to accomplish something like both holding a parachute or rolling a ball back and forth (you and son vs brothers then switch partners. As soon as you see them including your son in their play start to withdraw yourself from the game but be ready to step in if they start to exclude.