First, if you don't feel
calm and collected, you can fake it - it still works. My malinois is heavily dog reactive and always will be, and she's the first reactive dog I've had (and I've had very aggressive as well as very fearful dogs) whose actually gotten my heart rate going because she's not fixable and very quick to kick off. She has a hell of a temper and she will grab and rag, although she's never done it to a dog outside my group - but, still. A couple of months ago, we passed an alleyway on the field we walk on and through it and over the road, was a collie cross I've seen a few times - it too is very reactive but cautious. What it does is follow at a distance, barking, and its owners have no control. And for the second time, it was off the lead, crossed the road and came through the alleyway to freak out at my lot.
My heart was going like the clappers and I was shaking, but I faked it - cheery voice, confident movements, treats at the ready. As soon as I heard it barking I knew what was going to happen so I popped Ren on a lead then called every to me, and started walking off using that cheery voice to keep them near and jolly them along. It worked a treat - Ren didn't so much as look at the dog behind us and I was able to keep her calm until the useless owner eventually caught up with it.
Second, I would give him a few days of quiet to get over the shock before you jump back into socialising him. The stress of that attack will still be with him, so let it lower back down before you expose him to other dogs - this will give him a better chance of staying calm around them and he'll learn better, too. it'll also give you a few days to recover!
Then if you can find some already existing dog friends for him to play with, start there - familiarity, security - to remind him how good they can be.