The name on the birth certificate should not deter you from filing a child support and paternity case against the person whom you believe is the actual father. If he contests paternity in such hearing, then he is free to request a DNA test as part of that proceeding. The test will be made available to him and the result will be sent to the court. The court will find that he is the father despite the listing of another name on the birth certificate.
I think you should hire an attorney. An attorney can assist you to ensure that the father's income is correctly calculated. An attorney can also facilitate all communications between you and him for the purpose of getting all the information that you need to make a strong case in court.
I agree with the above advice, but would add the following -- you should not wait to begin child support proceedings. The court has the ability to backdate child support payments to the date the petition is filed, which means that your ex could be required to pay you back-support payments for the period of time between when you file, and when the court enters a support order. This is particularly important in your case, as it is obvious from your question that your children are close to the age of emancipation (18), at which time his obligation to support them is extinguished under Maryland law (19, if they are still in high school, or unless he affirmatively agrees to pay longer through a separation agreement). Depending on your jurisdiction (I believe you reside in Cecil County?) it may take several months for this to get a court date. One final thing -- in the smaller Maryland counties child support is enforced by the local State's Attorney's Office. You may want to google your local SAO, and get into contact with them immediately. Ordinarily, there is no charge associated with their services to collect child support on your behalf. The only time this may not be in your best interests is if your ex has a job where his actual income is difficult to prove. If he receives a W2, then the SAO is probably your best bet. If you have to demonstrate the existence of under the table or cash income (requiring bank records and possibly expert testimony) you're generally better off with a private attorney.
Finally, do not agree to a child support amount from him until you have been apprised by a lawyer (either privately or through the SAO) of the amount he should be paying you. You may be surprised at how high it is, and you wouldn't want him to be able to allege and prove that you both agreed to a lesser amount as being in the best interests of the children (occasionally a court will agree with the Defendant-dad and order a below the guidelines support amount, based upon the parties' prior behavior). That is not to say you should refuse any support he offers -- just don't affirmatively state (in writing or verbally) that you agree that should be the set amount. I purchased the same child support calculation software as the courts in Maryland use, so if you want to email me with some basic income and health insurance information, I can ball-park the support amount for you. Hope this helps!
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