Texas Child Support

I don’t have advice for you bc Idk anything about support but I would make sure to check with a lawyer or someone who knows more before putting him on you don’t wanna make a mistake and then regret it later.

You'll definitely need to talk to a family lawyer, you'll likely be appointed one through the court unless you want to splurge on a private lawyer.

You'll ultimately need to provide receipts and evidence of your financial contribution. Medical bills, copays, childcare costs, etc as those can be paid back in half to you. For the child that isn't his, you'd have to have had him establish paternity/adopt to be eligible for support with them or pursue child support on their bio dad. It's a whole other process if you need to establish paternity if he's not on the birth certificate.

I think you're going to need to make a strong case that you're separated or not together especially if you're living together... Short of you alone paying rent and housing costs/utilities I'm not sure they're going to award much to you given that he's been contributing to that much at least. Definitely worth asking, but do not lie about the situation and make yourself look worse. Come with the facts and start gathering receipts.

You need to talk to an actual professional for this one. Come with receipts, be open and fully honest about the situation.

You need to consult a lawyer. It will be expensive but this is not something to DIY.
You will likely have to move out prior to filing to prove separation. Are you married?

In Texas, even if you have never filed a marriage certificate, if you have lived together and represented yourself as married in a way that a reasonable person would assume you’re married based upon your statements, you could be considered common-law married. In which case the whole situation can become much more complicated.

The short answer is yes, you can leave him and file for child support. It does not matter how long he has been in your oldest’s life; if he has not formally adopted him he will not be responsible for child support for him. Is he on the birth certificates of the children you share together? If not, you will have to petition the court to establish paternity before they will grant you support.