Take the nappy off,don't put trousers or pants on at this stage,they get in the way.get the potty out.Sit him on it ten minutes at a time,ten off,then back on ten,ten off,just to catch it by accident.Stay with him at first,you can do puzzles ,books,whatever toys while he's sitting there ,keep it as friendly as possible.Be way over the top with praise and a Smartie if there's even a trickle in the potty.Just repeat no not on the floor,we do it in the potty.After a few days ,I usually find a couple of days,you'll see you're getting somewhere.By the end of a week you're pretty much there,but give another week for near misses and confidence before you start putting pants and trousers back on.It feels like it restricts yours and their freedom,but that part is only for a short while.
Use pull ups for naps,outings and bedtime but call them big boy pants or sleeping pants,and still keep reminding them about the toilet so they don't get in the habit of using them as a nappy.Don't be tempted to put a nappy back on,cos it confuses them,and you're right back where you started before you know it.
I've found this works with all my lot,usually in about a fortnight.You'll know they've really got control when you find them peeing every five minutes cos they know it gets a Smartie.
There's sometimes issues with number twos.That's sometimes frightening,I just make sure I'm with them and keep up the extra praise and of course number twos get two Smarties.
Oh Dettol spray,best stock up on that,but it's really no worse than changing a toddler nappy.
I'm waiting till the Easter Holidays and it'll be Matilda's turn.
Good luck,it's really not as bad as you think it'll be