'there is no telling' is an idiom used to convey the impossibility of knowing what has happened, or will happen : 'There's no telling how she will react.'
In your example, it means that having achieved what he has, having proved what he is capable of doing so far, then it seems like the possibilities for what else he might now go on to achieve feel limitless...who knows? It seems so wide open, it's impossible to predict - it could be anything!!
You will understand, this sentiment is born of an expansive emotional state and not necessarily reality!
- For Teachers