I know this probably gets asked multiple times but I'm stuck 
My Ds is 2 and a half. He has done random wees on the potty in past we've just encouraged but not pushed it. Today I decided it's time to start. I got him some big boy pants popped them on (he knows how to take them off and was taking them off and sitting on potty himself etc) put his potty down told him to go in the potty. When he needed a wee he just stood there, peed through his pants onto the floor and wasn't concerned that he was wet or anything. I just said oh dear cleaned him up no big deal but does this mean he's not ready and I should try again in a few weeks? He'll tell me if he's done a poo but wee's don't bother him at all.
He'll sit for short period on the potty and then say 'I done' there's nothing there.
I've tried to encourage him to sit on it longer and tried my best to explain what he has to do etc but I'm not sure he's getting it.
I'm just concerned because he's already 2 and a half and most kids I know were trained by now 