4 year old still not poo trained - pls pls pls advise

[Stangirl]

I have an older child who potty trained straight-forwardly without incident when she was ready around 2.9 yr.

My younger child was extremely potty resistant and after a couple of aborted attempts finally trained during the day for wee around 3.10months. He has now turned 4 but point blank refuses to poo on the loo. He just poos himself every day when he wants. He just says he "can't" do it.

He has actually managed it twice, once when he was next to a loo and we spotted what was happening and managed to stick him on just on time and once on his 4th birthday. He had said he would poo in the loo on his 4th birthday - so we kept the pressure off - and he did it! Then the next day just pooed himself again. When we said "but you said you'd poo on the loo when you were 4!" he replied. "I did. Yesterday."

NOT ONCE ON THE LOO SINCE.

Any advice gratefully appreciated. He starts school in 3 weeks and I have no idea what they'll do.

Handy advice in this thread. My DS is 3.6, so a late trainer. We mastered wees in about 10 days, and he enjoys helping to empty the potty, put his pants back on etc. However, he's had two poo accidents today alone. One of them was because he was so immersed in playing with my ipad he obviously didn't notice.

Trouble is, he doesn't seem to care. He's not bothered by the hassle of cleaning or having to stop playing to be changed. Other than lots of praise when he does well, I'm not sure what to do!