Are you the custodial parent of your son? If so, then file your own return and claim him as your dependent. You will have to file a paper copy of your return (not electronically). Include a note with the return that says you are the custodial parent.

But I'm wondering why your brother thinks he can claim the child? Did you all live in the same house, and did your brother provide financial support for your son? A few more details here would be helpful.