When do you have your child wipe themselves in a public bathroom if they went for a #2?

LO is now 3.5 years old and he has been wiping himself at home and at school after pooping. I do check on him afterwards to make sure he did a good job but at school they don't. It is all good because I have a toilet seat adapter at home and the toilets at school are kid sized. However, in public bathrooms I just had to hold him or he'll fall into that big big hole! I usually just wipe him after he is done. What do you guys do? Around what age would they be capable of balancing and wiping themselves, or I should start somehow training him to do that by himself? I am just not looking forward for him to fall into the toilet at all. That accident I can not afford...

Any advice is welcomed!