My son went through this ( and still does from time to time) but if you think about it...
Elmo ALWAYS talks in third person..."Elmo is going to do this, or that" You know what I mean? I really believe this stage is totally normal, a part of grasping verbs-tail end, and will pass soon enough. My son, who is three, does the same thing from time to time. I remind him that I, and WE, and US, and ME, and YOU will pass the ball. or YOU will eat and then play (to displace Elmo and bring forth real verbs in essence). I am sure this is all a part of learning, and I would not worry. He is probably learing this from TV or other kids watching TV, or BOTH -seeing he is in school and around other kids viewing the same kinds of things.
I would just have fun with it while you can. He is still little and cute, you know?? This time will pass very quickly, and then what silly things can you say to him? Suck up the young time while you can. Suck up the cuteness before it is too late!! Don't put him down for this. Love him for this!!! He is only this age once!!!!!!!!!!!!!!!!
xox
C.