All my boys had issues with pooing on the toilet.. I found they would poo around a similar time each day so we would go sit in the bathroom.. They woukd sit on the toilet and we would talk about how our tummies feel just before we needed to poop.. and basically we talked about gross things lol. How the body works ect ect

Eventually they just got it.. They did one poo on the toilet and went oooooo cus we made a big gross deal out of how it splashed in the water and all that.. Of course they wanted to do it again

Also we would tip thier poo from thier nappies or jocks into the toilet and joke around about the noise it made.. Gross yes but it helped them relax about it..