I would try putting him in a pair of undies and starting to take notice of *when* he is eliminating. Then you will be able t encourage him to use the toilet - put him on it - at the appropriate times.
Once you know when he is going, and his being in undies that DO get wet and aren't nice to sit in with poo in them, he may just start being more proactive. ...? Summer is a great time for some pants off action - another good way to actually see and know when he is needing to be put on the toilet.

I only suggest this as by this age, my eldest were at kinder, and nappy changing was just simply not done, so she needed to be TTd anyway, I didn't have the option of waiting for her to do it in her own good time. (She was TD anyway, so I never had to , but this would have been what I would have done - and will be doing with DD2 shorlty if there is still no signs of wanted to get out of the nappies!)