The last nerves to develop in a baby/toddler are the nerves at the bottom of the spine. (very often it is the first thing to go in old age) It is why the ability to walk varies.
Babies/toddlers can only walk when the central nervous system is fully connected to the legs. Then it is a question of balance. No one can get a baby to walk until then,
It is the same with bladder and bowel. All the training in the world cannot make a baby use a pot until the nerve connection from brain to bladder is complete.
Some babies and toddlers will perform on the potty, but that is sometimes more a spontaneous reaction that the brain working it out.
It takes time, a gradual process and you cannot hurry it. When the connection from the central nervous system - brain - to the bowel and bladder is complete, your DS will be able to go without being told.
If you worry about it, he will pick up on that and will connect anxiety with the potty.
If, when he is coming up to pre-school, he isn't able to know when he wants to go, some tests can be determined as to whether there is a physical reason for it.
You can find a whole lot more information on the internet and that may help you. If you ask your health visitor too many times, you may be taken as an 'anxious mother'. Don't quote me on that though. It depends how often you ask.