C., every child is different. So keeping that in mind go back to when your son was an infant. Not all the babies were getting their first teeth in at tha same time right? Some don't get their first one until 10 months old and some get them as early as three months.
Now, I would not be worried about your son's teeth unless you check out the following two things: 1) Did you ask your son if he fell down or got bumped in the face at school or anything that could have knocked them loose? Perhaps a good conversation about the possibility of getting hurt recently is due. 2) Do you know for sure that his teeth are getting two daily good cleanings? If you are helping him brush his teeth each day and night then and there is no other reason for his teeth to be loose, then I say Congratulations.
My son is 5, not quite 5-1/2 and his two bottom teeth in the front just came out over the past 2 to 3 months. I was also worried thinking it was a bit too soon and thought perbaps we weren't taking care of his teeth well enough. But, it's just the opposite. We take such good care of his teeth and he eats so healthy that his body was ready. He's got two big boy teeth coming in right now.
Hope this helped. Have a great day.