They usually start to show an interest in going to the toilet- taking off nappies, looking to sit on the potty etc etc.
I’ve got three boys who are all toilet trained, and to be honest, you’re wasting your time if they’re not ready, it just doesn’t work, and you just end up cleaning up puddles (and worse) and getting cross. My middle boy couldn’t start preschool in sept at 2 and 9 months when he was due to, he didn’t end up trained until the following feb, so only used one pre-school year. Having the second pre school year puts a lot of pressure on people in relation to toilet training.
hopefully your guy will take to it!