Hi, Kassie, are you pregnant now or is this just a question that is crossing your mind in case you ever do get pregnant with him? If you are pregnant, talk to your doctor, and have your boyfriend tell him what kind of "special needs" his father's child has and what kind of special needs his child has. It's hard to say if it is something genetic or just a coincidence without knowing the nature of the problem.
I don't think that theres any way of telling, all I cans say is take very good care of yourself and the baby(ies) when falling pregnant. they do test the baby for downsyndrome when your pregnant, I guess youll know then... but don't think that way, stay positive and pray for a healthy baby !