potty training

my 2yr old has been using the potty for 3/4 months now, and most of the time he is accident free, but he refuses to have a poo in the potty, so most of the time hes doing poos in his underpants (not exactly fun). i was just wondering if anyone knows of a way to try and get him to start doing them in his potty, when hes down his nanas house he goes no problem, but at home he refuses and its getting quite frustratring. thanx