My son is 6 month old and he wakes up from naps in the day, crying, sometime uncontrollable... he usually calms down and goes back sleep.
A sudden bout of 'wind' or teething pain may startle a child who is sleeping lightly during the day. He wakes up to find the pain gone and so falls back to sleep. An alternative suggestion is that he is dreaming (not necessarily unpleasant dreams) and wakes suddenly. Again, these episodes will pass.
If you are quite sure he is comfortable, warm and in a place he recognizes, it would be fine to go to him, perhaps stroke him, say a few soothing words and let him go back to sleep.