This is very common.
When your son is scared to poo and holds it in for days, he most likely gets constipated and that may hurt, increasing his fear even more.
My first thought is to make sure his poo is soft; enough liquids and fibers.
Some children have a fear to drop down in the toilet because their bum is too small for that big seat.
My 5yr old has a fear of the toilet because she’s very little (92cm). First of all a small toilet seat and a stool can help. My daughter still doesn’t do that well with that and I give her the option of using a potty to do her poo’s which works for us.
Another fear can be the fact that something of yourself (the poo) goes down the drain, the fact that it is gone now can cause fear (similar you might see fear for the vacuum cleaner, fear for the fact the bath water goes down the drain-what if I go down the drain) it might happen to yourself. This has everything o do with the magical thinking at this developmental stage and is normal.
Don’t respond in anger and don’t make your child feel embarrassed, but respond with patience and understanding and let him use a nappy if he wants to. You could place him on the potty with a nappy, then gradually he might be willing to let go of the nappy and do it on the potty and move to the toilet from there.

Many little ones seem to have a fear of that. One of ours did. Now he is better at going to the toilet for that than wetting his pants. Maybe the grandparents remember what worked with their children.

Thats interesting! It was good reading the comments on this!

My daughter was the same for a little while. She decided to put herself on the loo without her toilet seat and she dropped in, bum first. Put her off for a while. There are potties on the market, that’s what we used

This an old post and I hope you have had success with this toileting issue.

Maybe try a toilet seat and maybe reward him with a gift maybe start a reward chart.

Do you have a toddler toilet seat? He may be scared he will fall in.

I would be going back to the potty for poos there is no “splash” or make a game let him come in after a grown up has done a poo and let him flush it down etc

Hope you managed to find the answer you were looking for.

Maybe start a reward system?

Is your son going to the toilet now?

Put toilet paper in first so there is no splash. And stay and hold his hands. That helped my son.

How is he going now? Any luck?

There is a great interactive book called Elmo goes Potty, Kmart should have it.

Make a game out of it read him books on the toilet Buy a child’s toilet seat with steeps.Don’t put on a nappy.Go to the GP to get a checkup for your son & advice. Good luck

Thanks heaps ! Wondering this myself.

Hope things have improved since you asked

A potty for the moment and when he is a little older try the toilet again .

They will learn in time. U will b surprised. It jus takes patients

My daughter went through a stage where she’d do all her wees in the toilet, but would hide in her room or under the kitchen table to poo. It took a while but I just kept telling her that she has to do poos in the toilet too, and that mummy was getting sick of washing her knickers all the time, she eventually started pooing in the toilet