First of all, you shouldn't harbor any ill/negative thoughts to his second child. That is A) not your place, B) the child didn't ask to be put into the situation just like you and your son didn't ask to be put into the situation. It's only natural, as a parent, to want the best for your child(ren) but what the second child gets, really isn't your concern...the court made that decision for the non-custodial parent and the child's custodial parent.
In most states, the court order is what stands. If his job cannot support what is court ordered, then they will take the percentage in which they feel is appropriate (some states say that non-custodial parents can pay out 70% of their pay, leaving 30% to live off of...just an example) and they will divide that percentage amongst the custodial parents going along with the court order. So if she has a higher payment than you, then of course, she is going to get more of that percentage than you are. What cannot/has not been paid will get put in arrears. I would definitely go back to court and get an order for arrears. Then he'll have another order for that.
In my situation, the non-custodial father was ordered $350 in child support and then $50 in arrears. Do I get that $400 a month? No. I get $184.62 bi-weekly which equals out to be $4800.12/yr divided by 12 (months) = $400.01/month. So the numbers don't have to equal out monthly...as long as what he's paying annually divided by 12 equals out to be what is ordered.
Hope that wasn't too confusing.
Check with your state and see how often can child support payments be modified. If you haven't had any modifications and it's been the allotted time required for a review, take his rear end back to court, where you both will have to fill out another financial affidavit and see if he is eligible to pay more. You have to be careful doing this, though. If he has taken a pay cut since the original order, this can be to his advantage because he can be required to pay less.
I'm a single mom of four and I have been through this multiple of times...it's the same BS cycle every time.