No Poo for Three Days

[Fayzal]

I started potty training my 2yr 8m old on Monday. He's doing quite well with wees, had quite a few accidents on Monday but only one little one yesterday and so far none today. But!! he hasn't had a poo at all since he had his nappy on which was Sunday. He's had nappies at night and nap times and still doesn't seem to poo even then. When I ask him if he might like to use the potty or loo with his special seat on for a poo he says yes but then jumps off a minute later saying he's all done. Should I be worried about it storing up for three days? I don't want it to hurt when it does happen and scare him about pooing out of his nappy. What shall I do? Go back to nappies? Carry on regardless? Take him to the doctor in case its actual constipation? Help!!