My wife had two boys to her ex but her ex will not except one of is sons as is yet he looks more like is father than is other son does so my wife wants to no if its possible for her eldist son to take a dna test with his brother to prove that he really is his brother.

Should be possible. But all it would prove is that both boys have the same father. Her ex's response might just be (a) I don't believe your test result, or (b) that means neither of the kids is mine.