It is difficult. But it’s difficult for parents of all kids because there’s no exact timeline and it’s frustrating and it seems endless when you’re doing. Just know this that he will eventually get it.
My other advice is try as hard as you can to instill a complete routine. ( from walk in the bathroom, shut the door, undo button and zipper … all of the components right away. l don’t let him walk around without clothes on or leave the bathroom not dressed.. because I’m having a devil of a time having my 13-year-old son understand the concept of nakedness and privacy because we just let him do whatever he wanted to do ( some of it was to make it easier on us. Just leaving the bathroom door open to get in out of it if you needed us and being distracted and having to run back up to get him a towel all of this ) do it correctly with all the social expectations from day one and don’t vary. it will make things so much easier on you and say other siblings but also it’ll keep him safer when he’s not with you at school in different things he knows exactly what is socially expected of him.

Please try this as soon as he finished eating and drinking something put him on potty or when he makes a face to put him straight on potty. Watch what time he goes toilet and use that