I don't think anyone can answer that with certainty. At one year, he wouldn't have had words to help him interpret what happened. However, six hours alone with no adult responding to vocalizations or cries undoubtedly left him with an overwhelming sense of fear and helplessness. The impact of that experience will have been influenced by whether he has had good loving care since the episode.
Chances are his current behavior is more related to his struggle for autonomy and control—behavior that comes with the territory of being 18 months old—than to a definite residual of the earlier tragedy. I would deal with his behavior as though he was simply going through a growth period when such behavior is the order of the day—that is, put him in time-out when it happens, comment that hitting and kicking are not allowed, and pay a lot of attention to the child who was hit. Then, when he is behaving appropriately, be sure to give him plenty of love (including physical touching and holding) and attention.
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