I feel your suffering.
What is he in at the moment?
There are some aged 2-3 school trousers about. Have you found him anything that small? M&S have quite a few.
Personally, we have always had to go for button waists in everything except shorts. We need the length of the age. So in reception, DS1 was in 3-4 trousers initially, then 4-5, as he needed longer trousers. I've now discovered longer length trousers, so he is now in 4-5 tall, with elasticated waists pulled in. He spent the summer in age 2-3 shorts (his brothers...) They didn't need the wasit pulled in!
We also have trousers sitting quite low on his hips, which help with getting them up and down. The button position takes some adjusting - there is a very fine line between them falling off and being able to be pulled down. I'd suggest letting out each side by one button hole, and see if they will stay on the following day. If success, let out another button hole til you can see they are too loose as they are constantly being hitched up. then tighten one hole, and see what the toileting is like.
It gets easier when you can size down with long legged versions. Worse when they can read the numbers and say "Mummy, I'm 5. I'm not wearing 3-4 clothes". Solution. Show him the new clothes in the larger sizes, and then make him wear the smaller ones......