First of all, I absolutely cannot imagine having to go through what you must be going through, my heart goes out to you for that.
I don't know where you're from, but assuming the adption is legal and finalized, I expect the biological parents would have very little claim to regain parental rights years after the fact. But I shouldn't speculate on probabilities, and it's not what you were asking.
If you haven't already, now is an excellent time to talk with your child about the fact that he is adopted. It won't have been for nothing if courts rule in your favor, this is still a part of your child's identity, that you'll at some point need to address, and sooner is always easier.
Keep in mind that it is not in the best interest of your child to build antipathy towards the biological parents. If he is indeed returned to them, this would be a terrible starting point for rebuilding that relationship, which would hurt him more than them. So be neutral, within the bounds of honesty.
If true, you may for instance talk to him about the fact that he has another pair of parents, who loved him dearly but could not take care of him. If true, you may tell him that they are now better capable of providing for him, and want to have him back in their lives.
I wouldn't shy away from showing him how you feel about this. Be careful that he doesn't end up being your caregiver - children at that age can take on way more responsibility than is good for them. It's absolutely fine to cry in front of him, or to say you're terrified, but I think it's also a good idea to follow that up with reaffirming that you will be OK, and that he needn't be worried about you. I think it would be confusing for him if you weren't appropriately upset by this, so that's a blanace you'll need to find.
Talk to him in ways that you can live with reardless of the outcome. Honesty is key, it's tremendously difficult to depict a romanticized version of reality that won't backfire in one or the other routes.