Generally, child support is based on the income and assets of the parents. Your earnings should not be relevant to what he is obligated to pay.
There are ways that his child support obligation may impact you if you marry.
First, he has a child support order and the court will not be concerned over his expenses when enforcing or modifying the child support order. Therefore, he will only be able to contribute to your common household what is left of his income after taxes and child support.
Second, if you and he have children and later separate or divorce, the court will not order him to pay you child support in the same amounts as for the children who already have an order. The Connecticut Child Support Guidelines use a percentage based on the idea that the most expensive child is the first, which is usually true for children all born of the same father and mother. However, this causes inequities in situations like yours. The Guidelines state the amounts of support couples should contribute between them based on their combined net weekly incomes. The non-custodial parent pays the custodial parent a percentage of the guideline amount determined by comparing the payor's net weekly income to the combined net weekly income of both parents. For example, if the custodial parent makes 40% of the combined net weekly income of both parents, the other parent pays 60% of the Guideline amount.
The Guideline authors believe that each subsequent child is less of an expense in terms of a percentage of the parents' incomes. For example, if the combined net weekly income of the parents is $2,000, the Guidelines say that the cost of 1 child is $319. If those same parents have two children, the Guidelines say that the children will consume $423 per week. Note that the first child costs $319 and the second only $104 more. The Guidelines add only $44 for child number three, at that income level.
Assume that you make as much money as the mother of your boyfriend's three children and he is still supporting his first born children when you seek an order of child support from him. Your child support order would not be calculated using $319 per week for one child as the other mother's was. Since your child is your boyfriend's fourth, you will probably get something around the Guideline amount for a fourth child. Using $2,000 net combined weekly income as the example again, we see that a fourth child increases the support amount from $467/week for three children to $521/week. That is only $54 and, remember, if you and your boyfriend each are making $1,000/week net, his share of the child support obligation would be 50%. That means he would only have to pay you $27/week for your child, until his pre-born children start to emancipate.
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My answer is not legal advice and does not establish a client/attorney relationship. The question may not be a complete or accurate description of the problem and there is no chance to ask a follow up question. It is impossible to give complete advice without a thorough discussion of the facts, such we would have during an initial consultation. Further, laws are different from jurisdiction to jurisdiction and frequently change. So, please, do not act on any information provided without consulting with a lawyer licensed to practice in your jurisdiction who has experience with the kind of issues that concern you.