He has a responsibility to pay maintenance to your child. He isn't legally responsible for his girlfriends child.
Has he still having regular contact with your son, and have you spoken to him about the maintenance? whatever you do don't stop the contact ~ if it all goes to court you won't be looked upon favourably if you stop contact...and I have to say a big 'good for you' for keeping this going ~ so many use their children as pawns in the maintenance game and it isn't right.
If you are in contact with your ex and haven't discussed it with him, do so now. Tell him you and your child need financial assistance, and regularly at that. If he isn't interested, then a court order is probably the best way forward. You can apply to the family courts for maintenance (you can work out a figure and it can be discussed).
The only other option is the CSA ~ I don't want to recommend them as I have had nothing but trouble from them!
The best way is to do it privately, but obviously this hasn't worked out. It may make your relationship with your ex worse, however rest assured you are doing the right thing by your child which is all you need to be concerned about.