Will I have to repay child support money to my ex husband as I have just found out that he is not the biological father to my child as he has done an on-line dna test without my or my child's consent. My concerns are that I am in no financial situation to be able to do this and 100% believed that he was the biological father. My child is 10 years old and he has never questioned his paternity

You really need to talk to your CSA contact, but an unconsented online DNA test holds no legal precedent.
If he has never questioned the paternity, why has he chosen to take this step ?

What country is this in ? From your nick Im guessing across the pond ?

In the US, a "online" or any lab that is not approved by the court is worthless. And the law varies,
1. he will have to keep paying until he gets the child support order charged.

2. in many places, he may not even be able to get it changed, even if he is not the father, if he waited too long to challenge the bing the father.

So you will need an attorney if he does not keep paying, or if he challeges it in court