Take his nappy off and have a potty nearby. Note DS reaction when he wees, does he try to get to the potty or does he just do it without any apparent awareness that it was going to happen. If either of these, he may not be quite ready. If he puts himself on the potty, and does something, he's ready, because he knows what to do and where to do it, and can hold on until he gets to it.
Don't confuse him with pullups, as they feel just like a nappy and he will use them like one. Pants are best, or loose shorts if you need to go out. You could also make use of swimming shorts, the loose sort with the mesh pants-like lining, they won't contain an accident but they wash and dry very quickly. However, with my DS I did used to put a nappy on him for long journeys in the car because he always fell asleep! This was in the days before seat liners were introduced. You could sit your DS on a plastic carrier with a towel on it, if you thought he would just use a nappy while awake.
I've just started and I do bare bums... I think pull ups are too much like nappies and too 'comfortable' and confusing
I'm having success with wee's but not 'letting go' yet to do a poo..
Am now going to try and use underpants around the house/garden to get him use to pulling them down when he needs a wee.. he's only 2.3 but has seen it with his 3yr sister so thought I'd bite the bullet.. now certainly the best time of year for a bare bum...
Short car journeys we sit him on a towel!!
Just back from the park - accident free - he went 'commando' - just his shorts - no underpants, no nappy... during the picnic he saw another little boy having a wee stood up at a tree and he wanted a go!! bless.. he actually did a little wee..
lots of rewards, big fuss and bribery seems to help too