Let's call the OP "A" and the other bloke "B"
If the mother of the kids chases "B" for child maintenance for both children and "B" denies that the first child is his, then the mother will presumably chase "A"
"A" should deny that the child is his because of the DNA test.
If the Mother then says that the test was wrong, then "A" should go directly to his solicitor and take legal advice, which I'm pretty sure will be informing the Police of this matter
There is also the likelihood of criminal charges being brought against all involved in the fraud and who knew about it, Mum, Mum's Mum, brother and "B" with a very strong possibility of people going to prison
If you do have to talk to the Police, they will absolve you of all responsibility as you're the person who has been lied to. If they ask why didn't you come forward as soon as "B" contacted you and told you about a fraudulent DNA test, you just viewed the entire split between the Mum and "B" as being very bitter as it was when you split from her
My advice to the OP is very much as has already been said. Do nothing until you are contacted by the mother of the child. A few weeks or months will not affect your relationship with the boy if he is in fact yours