The courts do not generally take into account the fact that the person paying child support has other children, unless he is currently paying child support according to a child support order. If he has made child support payments for his oldest son previously, try to get proof of the payments so that the amount he has already paid can be applied to any child support arrearage that may be established.
There are a lot of legal issues contained above and you should seek legal counsel - even if simply for a consultation. The Paternity action can address his concern that he may not be the biological father, and if so establish paternity. It will not address other issues of rights and responsibilities related to being a parent. Other children born by your husband, without a prior Court Order for Child Support, will not be contemplated within a Child Support award. That said, even informal payments that can be accounted for will be considered by the Court in providing credit for any arrearages sought in the action (which is routine).