I started to train my boy using the toilet bowl (wee wee and poo poo) when he was about 2.5 years old and it took about 3 months to get him willing to do so. Here is what I did and hoped it helps:
1. Keep telling him that mommy, daddy, (whoever is important to him) using toilet bowl as we are all adult as he always wanted to be an adult
2. Has the helper to tell the same things .....
3. When we did go toilet (during weekend), we'll tell him about it and praised each other greatly on it
4. Put him sitting on the toilet bowl (wee wee) for 1 to 2 minutes before showering and before sleeping at night. Has his favorite teddy to sit on a can next to him.
5. If he tried (but nothing happened), big hug and a lot of praised for making such effort
6. If he tried and able to do so, big announcement at home and everyone praised him
6. Bribery - if he 'poo poo' on toilet bowl --> he can have ice-cream with daddy initially; As soon as he was able to do it few times, we stopped the offer.