momamia - personally, I found the only way to do it is just to spend a week near the house/garden in the summer and just keep his nappy off the whole week. Buy some antibac wipes, floor wipes, carpet shampoo and a big bag of Magic Stars! Tell him that from now on he is a big boy and isnt going to wear a nappy, just put a t-shirt on him (that's why you do it in the summer!) and let him play around the house/garden etc. If he does a wee he will feel it, tell him that he must sit on his potty to do a wee and that if he does he will get a chocolate magic star. Have potties to hand in several places or keep moving one around so he can see it and remind himself. He will learn to associate seeing himself wee as an accident and realise the feeling he gets just before he does a wee and by day 3 will probably be getting to the potty first.
When he does a wee on the floor dont shout at him or get cross, just say "You've done a weewee, thats for potties" and plonk him on the potty, obviously he wont do another one straight away but he'll learn to associate it. At 2 1/2 he definitely will have the ability to associate the two things.
When he does do a wee on the potty, go crazy, shout, clap give him a great big hug and say what a big boy you are! give him a magic star asap.
It's basic behaviour reward stuff! Ignore the wees on the floor, pretend they dont bother you (it wont be for long I promise!) and go overboard with the positive reinforcement when he wees in the potty.
Not everyone is going to agree with me, but it worked a treat for dd at 21 months, she was dry day and night in 3 days - yes, its a hellish few days but if they know they cant go back into nappies they soon get the hang of it.
Go and choose some fab "big boy" pants (even better get him to go and choose big boy pants with your dh/dp) with his favourite character on so he wants to wear them instead of nappies. Take him with you when you go to the toilet and ask him if he wants to sit next to you on the toilet, also, let him watch dh wee, I know you lose your privacy but kids need to learn!
Hope whatever you choose to do works for you! Good luck