The main problem here is not that your ds can't cope, or is failing to stick up for himself, is it? In fact, he seems to be coping perfectly well. He had forgotten about the scratch and was as far as we can see happy. If that's not coping, then I don't know what is.
Let's shout it loud: HE IS COPING WITH SCHOOL!
The person who has difficulties is your dh, is it not? This is more likely to be about him than about ds. For some reason, he has fears, either of school as such, or of this particular school, or of other children, or of the whole concept of beginning to let go of ds.
Can you point out to him (gently) that he created the problem with the scratch; that your ds was fine until his Daddy went and told him that he wasn't! And that if he carries on doing this, he risks undermining your ds's confidence and turning him into a frightened and unhappy little boy. Is that what he wants?
IMHO we have none of us the right to give our children our own fears- they'll have their own separate burdens in life, and probably ones we hadn't even thought of; they don't need ours. My Dad who has a dog phobia bravely let me play with (and even feel the puppy teeth of )next-door's dog; I am still grateful to him- dogs have given me a lot of pleasure in life. I would have hate to have had his fears instead.
The other thing you need to tell your dh is that if his ds perceives him to overreact and interfere to much, he is likely to stop confiding in him when he gets into his pre-teens.
Personally I think if you start talking to ds lots about coping mechanisms at this point, you will give him the message that you don't think he is coping as it is. There's no evidence of that, is there? Leave the coping talk until he shows you (without words being put in his mouth) that he is unhappy.