I did DS at 3 (now 5) for exactly the same reason. All the following was gleaned from Mumsnet at the time and much of it from the legend that is/was Cod who was queen of potty training.
So I took a Friday off work with Dh having the Monday off in June/July.
In the run-up to it, I started getting him to use the potty before the bath, no stress just practising. And I bought one of those loo seats which he was very intrigued by.
I bought lots of pants but let him choose them - he found it all v exciting.
Then on the friday, weather was good and he was v excited about the big day. Potty lived with me. One up, one down stairs, tons of cheap loo roll for accidents and spillage. Spray detol thing for quick cleaning of potties and again cheap loo roll. Agreed a treat with DS for each pee in the potty. We chose one choc button. Immediate reward and possibility of several a day without it being a packet.
Started with nothing but pants, after four accidents I abandoned that and took all bottom half clothes and went into the garden. That did the trick. First pee, was a total, what's that what's happening form DS. Then he started to click that the feeling meant the willy. Gave him plenty to drink too to increase frequency.
Ker-ching. Within three days accidents were way rarer. Moved to pants on Day 2 and 3. He was very excied on Day 2 to get up and put pants not nappy on. We didn't go anywhere for four days and stayed at home so no stress at all. Checked with them him fairly often if he needed one, but didn't obsess. he actually peed about once an hour at that stage I think. After that I went everywhere by car, took potty everywhere and cleaning kit and praised, praised, praised. Never ever cross about it. Washing machine went a lot. Phased out buttons by day 3 as much as possible. But always give treat quickly after a success.
Think him being older helped a lot. By the end of the summer it was all sorted and he'd settled into it. Didn't need the potty to go everywhere and had got into using public loos. Poos came as part of the process. When he got to school never a problem as he'd been at it for 2-3 months, not a "new" thing so well entrenched habit.
Now night time, I still haven't cracked, so any advice welcome ...