I have a beautiful grandson in germany who is 3 and a half and not quite potty trained. my son was potty trai?

my son was potty trained at 15 months because i was travelling through europe, just with him on my shoulders,,,and a backpack, and, the english pound was the highest it had ever been. $2.40 a pound. that was double and a half the dollar. so, diapers were 12 for $10 or so. i couldnt afford that, if we wanted to eat!my son is soooo smart. i said, there is the toilet. go pee in it. and, he did,,,,right off the bat , he got. it. well, freud might not have liked this early potty training. he believed that if potty train was initiated too soon, it could have lifelong consequences to the individual, and, they might get "stuck" in one of 4 stages...this, being the anal stage. well, i think a child should definitely be potty trained by 3...with minimal accidents. my son just divulged to me that HE wet the bed till he was older. it this from nerves, or biological in nature? maybe some children dont have a fully developed bladder,and cant "hold it in," or dont know when they have to go to the bathroom? do you think a child should be fully potty trained at 3 and a half? on a long trip recently....10 hours....my grandson pooped all over the carseat, himself, the car. i nicely suggested to my son that he put on a diaper for the remainder of the trip. he bit my head off, told me his son WAS potty trained, and told me to mind my own business and not tell me how to raise his son. whatever... thank you