My exhusband had our son emancipated and he was behind on support. My question is interest added on to the back support until it is paid up?

Only if you took him back to court for non-payment and the judge entered a judgement against him.

Mostly that depends on where you live. On a personal note Im against interest on child support as its putative and not good for the child in any way.

In most of the areas I have been at, there is not, only added if the one party takes the other backs to court and the court orderes a new payment plan, ( and not always then)

I will assume some states may have it automatic,

But the details should be in your court order,