L.A.
My son will be 4 in two weeks. And he STILL isn't fully potty trained.
If he is wearing big boy underwear, he will usually use his potty chair, but if I don't have a clean pair and he has to wear a diaper, he will not use the potty chair. Oh, I also don;t expect him to use public restrooms, yuck!, so he wears diapers when we go out too.
Boys seem to get lazy and comfortable when the diaper is on!
Go ahead and put him in the big biy underwear, even if you have to put the vinyl overpants on top just in case. Then, every 30 minutes, put him on the potty chair. He'll get the hang of it, and when he does have an accident, he'll realize how uncomfortable it is.
Give it a try and if he isn't ready, then put it off for a while. But you never know until you try.