You just have to decide you’re going to start and get on with it.
-Chuck the nappies, they are no more; get night time pull ups for bed to differentiate
-Plan to stay home for 3 days
-Get him big boy underpants with a character that he likes
(The naked method, from the Oh Crap book is aimed at much younger children aged 20-30 months so I would avoid)
-Push all the fluids, especially those he might not normally get like squash, juice, smoothies, ice lollies
-Take him to the toilet with an insert seat (he’s too old for a little potty) every 30 minutes and do whatever it takes to get him to sit for a few minutes- books, fidget toys, TV on your phone
-A small reward eg smartie for anything produced
-He has to clean up any accidents eg wipe up puddles, pants in the machine etc - don’t be cross with him, just say ‘oh dear’ and make sure it’s more inconvenient than going to the loo, he’ll soon catch on