Hi,
If your son is named on the birth certificate of the child then it is up too them to prove that yur son is not the father.
With regards to the DNA side of things go to the CAB as they will be able to put you in touch with companies that can do the tests and they will be able to tell u what dna they need.
(please don't take this the wrong way) if your son has ever been arrested then the police may have his DNA on the database, or if your son has been in hospital at all recently then contact them as they may have it especially if there was a post mortum done recently as they may still have tissue samples etc...