How do I know when he's ready

Rhys is 21 months and we trying with the potty training but getting nowhere, though we are taking it slow still, basically sitting him on the potty throughout the day. He knows what the potty is for as he sits his teddies on and says poo poo or wee wee, but doesn't seem to have any indication when he does anything in his nappy. He likes going on his potty but so far we've only had 1 wee on it. Just after some advice really of how to go about getting him on it