There will be difficulties like this with ANY age children that you care for.
Before 3, kids don't really play "together" anyway, just parallel to each other. The boys play with each other because that is what they are used to. As they all get to know each other, and as your son gets older, they will play as a group more and more.
Why are they saying "no" all the time? Is there anything that perhaps your own boy can start working on, like not taking toys or things like that? If he is allowed to do that stuff because he is "little", then it's not really fair to expect them to play with him. It's not fun when there are different sets of rules for everyone.
I would say give it some time. If there are behaviours that need correcting, you can do so, of course. But also keep in mind that he could be having a hard time sharing his mommy right now. That will not change with getting new children. In fact, switching kids out may make it worse.
Best of luck!!