How to potty train boys?

Hi i need help with potty training my 4 year old boy he refuses to wear undies he cries for his Nappies, and when i leave him undies or bare he will no1 or no2 anyway, his so afraid of the potty and toilet with baby seat on, he gets hysterical when we tell him to peepee on the potty, i tried treats doesn’t work, anyone experienced this with a 4yr old please help