Jaydee went through a funny stage like this... In the end, we could see that he was posturing for a poo and hiding under the table before asking for a nappy.  We left his potty under the table, (even though it sounds gross and he had generally stopped using it).  After a day or two he felt comfortable going back to the potty to poo... We slowly moved the potty closer and closer to the bathroom until we got it just outside the bathroom door, but in view of the tele.  Then by the end of the week, he was using the potty in the loo; then he just ditched it and went back to using the toilet all the time.  We found that by not fussing and letting him do his own thing, he transitioned back very well in his own time.  I still don't know what caused it, maybe he got a splash back or something?  LOL ; )  Good luck with it, I am sure it is just a stage, so be patient.

 My boy was similar.  He would save it up for when we put his 'night-time' nappy on.  

My boy said he liked the feel of it in his nappy.  He also thought the poo would miss him if it went down the toilet.

So we talked about the fun ride the poo would have if it went down the drain and into the ocean.  He did a poo on the loo yesterday for the first time in about 4 months! We waved good bye to the poo and gave my son treats.

It is normal for them to revert back to the nappies etc.  It doesn't mean that anything bad has happened.

I would talk to your boy and find out why.  

He is still young and I wouldn't worry too much cause it will come together aventually. But I would do for the time being is let him do his poo how ever he likes only because my neice who is now 8 has a lot of troubles with her bowels and toilet routine due to holding poos in.

DONT BLAME THE SIS!

Kids regress naturally especially when it comes to potty training and nothing bad has to happen. Just like you will find yourself repeating everything you say for the next 18 years potty training will have to be done more then once.  Some children don’t get it down until they are older and that doesn’t mean anything about their development (Boys are especially hard to train) So keep fighting the poo fight - give him some prunes with applesauce -if he likes applesauce- and keep him hydrated as this will make him go often. He will get the hang of it again. No worries! Whatever you do try not to get frustrated with him because that will cause anxiety.