Just Turning three year old boy stuttering with first word of sentence suddenly?

He started talking VERY early and has been speaking in full sentences for a long time. He speaks very clearly and can be understood by all. Over the last few days he has started with stuttering the first word of a sentence, for example....."I, I, I. I, I, I, I, I, I, I, I, like this song." He gets stuck on the first word and sometimes won't actually finish the sentence cause he is stuck. I find this very strange! I could understand if he was just starting to talk but he has been speaking fluently for a long time now. What should I do? I also have two older children who are teasing a bit. Obviously I am putting a stop to this but is there anything I can do to help him get over this??