A., please keep talking to him. Don't overdo it or harp on it but keep the lines of communication open so he feels he can tell you things.
I suspect that possibly he really was hit by these kids, and the second story about falling down was his attempt to cover it up. He may fear that you are going to tell the school and he will then be bullied worse. Kids this age are very concerned about what their peers think and very concerned to appear tough and able to take anything other kids dish out. I think when he cracked and tearfully told you he was hit, that sounds likelier than when he had had time to think up a cover and told you he just fell down.
I am not saying he's an intentional liar; I"m saying he may be one very scared and confused little boy.
This is the time when it helps to have good communications with his teacher. I would go (in person) to the teacher and talk to her or him alone (without your son there!) and explain both tales. Say that you are concerned that he may be covering up being hit. You can ask the teacher if she or anyone else witnessed or mentioned a fall, too.
Ask the teacher to be frank with you about whether there are boys in the class who seem to be excluding or picking on your son. Any teacher these days will be very cautious and may not give any names, I expect, but put the teacher on notice that you suspect your son was beaten up and expect not to hear any further reports of this.
The teacher and the school DO need to know about it, regardless of whatever he said later about falling down.