Forget about his age for a minute. Have you scheduled a meeting with his teacher about this? I mean a serious sit down with just the teacher and the parent(s). You need to get specific information from the teacher such as:
-Is he the only child having trouble focusing?
-What do you mean by "his focus" - what examples can you give me?
-Is he keeping up with class work?
-Where does he fall in class rank? (Is he in the middle, in the bottom third, at the very bottom)
-Is he learning the basics?
-How are his social skills, does he have friends?
Talk to your son about this without seeming angry or scared that he is getting left behind. Just ask him what he likes about school, the teacher, the kids. What is his favorite part, what is hardest, what is easiest?
You need to get a better idea of how he is functioning in the class. If the teacher and the principal think it would be good for him to repeat the grade, ask them what the advantage would be for your son. Once you have as much information as you can get, you will be better able to make the right decision.
Your son will take his cue from you. If you decide to let him repeat, talk to him openly about it. Let him know it is your decision to make and you are doing what is best for him. Ask him to trust you and then make a special effort to invite classmates over, etcetera so that you know he has friends. If you decide it is best for him to continue on to third grade, you may want to look into a tutor or a study partner, and you may need to spend more time with him yourself on homework and classwork. If he sees that it is important enough for you to do with him, he may sense that he has to make it a priority.
I have three grown children and it was my experience that the two boys -both were among the oldest in their classes- had a much easier time in school than the third who was among the youngest in the class. The most important thing is that they learn that school isn't always easy, and you just have to find a way to do your best.
Good luck. Keep asking questions-until you get the answers you need.