Potty training HELP

My son is 3&1/2 and we have been potty training for what feels like forever! At my house he will go fine, but i have to tell him to go use the potty or he’ll go on himself. At his dads he makes someone go with him. And at the sitter he just doesn’t use the potty. I have praised, rewarded, got a urinal with the spinner for him to aim at. I just cannot get him to go without being reminded.