I realise that you want nothing from your ex but his acknowledgement, and its sounds like you have very good reason. Is his name on the birth certificate?
How would you feel about reporting him to the CSA? Not particulary get any money but to get him to face the facts, that he has a son.
How hard have you tried to track him down? Have you tried directory enquires or doing some serious digging on the net.
I think you need to tell him directly what is going on with his DS, ex mil can be vicious. Then if he wants nothing to do with your DS you have it from the horse's mouth and you can tell him that he is going to have to face up to his responsibilites even if it is simply finacial.
Its possible ex mil is being so nasty as you ex partner's new partner has no idea, perhaps she would see your ex in a totally different light.
I really feel for you bouncy and to be honest my rage is building.
My DH never knew his father. In an attempt to get some acknowledgement he and his mum took him to court (this is in the USA) they had the paternity test and despite the positive result he stood up in court and told my then 12 year old DH that he was not his father. And that even if he were he refuses to acknowledge him as he was the result of a gang rape.
My DH has never come to terms with this, and what makes it really hard is that some 14 years later his mum and dad are now 'dating'.
I think that it is more important to sons than daughters to have the acknowledgement, even if there ends up being no deep bond.
I hope I haven't ranted too much, but I hate mils and men who are s**ts!