I was told, that pooping while standing is different than when sitting down. Different set of muscles etc.
Another Mom told me that, and her Pediatrician told her that, because her son was doing what your son is doing. But this is VERY common.
In time your son will poop, while sitting.
Just don't pressure him.
Because some kids, will start to withhold their poop, and then will not poop at all. And this will cause internal medical, problems. Encopresis for example. Or bulging bowels and constipation, etc. And once a child gets constipated because of it... it is VERY hard, to undo.
My son was like that too. And one day he went poop in a toilet.
My daughter was like that too. And we even had to take her to a Pediatric Gastroenterologist, because, even if we did not force her about popping in a toilet... she started to NOT go poop, at all. And withheld it. And got Constipated badly. The Doctor told us: it is a physical AND emotion based thing. Some children, get stressed and have anxiety or "fear" about pooping in a toilet. He said, just let your child poop, even if that is in a diaper for now. Because you do not want medical bowel impaction problems. A child, WILL poop in a toilet, when they are ready. It took a LONG time, to undo the constipation and poop withholding problems, my daughter had. I am talking, months.