hi, i've just done the potty to toilet bit with my just gone three year old. i used the promise of easter bunny bringing him a new toy car, (easter bunny made a huge impression this year , apparently!!) I so know that bribing is wrong but it SO worked, had him out of nappies on to potty then toilet and even out off bedtime nappies in a whole 2 days!
However, my first son was gone four before he used the toilet for poos, he insisted on a nappy for a year between weeing on the toilet and pooing on it.
My daughter however, had a few weeks between each stage .......... just goes to say how different they all are and WILL do it when they are ready...(unless your grandson REALLY wants easter bunny to come extra early!!!