When I go to court for support, will the court deduct what he pays her or will they take into account that they've been living together for 8 months?

The court does neither. It will take into consideration that he has two other children for purposes of deviation. Do inform the court that they are living together. As to your older child, the child's disability payment is the father's support.

Your support is not based on whether he has more spendable income. Your support is based on his net income as compared to your net income. The only question is which column of the support calculations should be used as he will have three children to support when you deliver. Should the three column be used? No. Your child is the only one between the two of you. The one child column should be used and then he should be asking for a variation on the amount based on his need to support two other children. The could will make some adjustment, but not much, particularly if the court is informed on his current living situation. You really do need some help with this. See if your court has a facilitator with whom you can meet if you do not believe you can hire an attorney. Some attorneys may consult with you without being hired for a fee too.