If my husband & I file jointly & he wants to claim his son, does that mean I'm claiming his son too?

Yes...but it just means you get more back and that you supported him in a certain way last year.It means nothing of what is required in the future in the way of residential support or whatever...Most people clamor to claim a kid because it is financially advantageous...you could file separately but that probably would cost you money......I'm quite positive your husband is aware of what you need to do to get the maximum and IRS get the minimum.....