kikki,
you must be feeling pretty bitter right now, and I dont blame you.
BUT to keep things in perspective:
he has got 2 children in the UK, therefore humanity dictates that he should be free to come and see them and work here. Thats just logical, whether he lied to you or not.
2nd point is, there are tossers all over the place who lie to women, hit them and abandon them, and try to get out of paying maintenance. The visa thing is only one more aspect to it all.
There are many people who will marry for dual reasons, ie a visa and a genuine marriage. They will take more risk, because they know that if the marriage doesnt work, they still have the visa. What I am saying is, that even though the visa may be a motive, you cant always say that it was 100% the motive.
It is deeply wrong, imo, to increase the waiting time for a permanent visa, because of a small number of fraudsters. It may lead to people staying trapped in unhappy marriages, because they will lose their job and home and country if they divorce. Some of these people may be women, btw.
You can write to the Home Office and explain why you feel conned, and tell them the evidence that he tricked you into marriage. I am not sure if there is provision for them to take back a permanent visa if they believe that it was gained by fraud. But I would only do this, if I thought that he would never contribute anything positive to his children, and that he was a continuing danger to them or to you.
You should definitely be able to get maintenance out of him, just the same as for any British resident, citizen or not.
Sorry if this sounds unsympathetic. I really do feel for you, and hope you have better luck in the future. It is very hard to give everything in a relationship, and then get kicked in the teeth. At least you have come out of the relationship with a son, and it sounds as though your ex doesnt value the love and respect of his son that is his loss in the long run.