Unless you EC (which is what I think the pp was describing), there are several things a child needs to be able to potty train:
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1. Awareness that they are going/need to go potty
2. Ability to hold it long enough to get to the potty
3. The ability to let it down once they're sitting on the potty.
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#3 is often the last to develop. If your son is aware that he's peeing, it can't hurt to try. Some kids are ready at 18 months, others not until 4. Ds trained (with bribery) at 3 1/2. Dd self-taught at 2 1/2.
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I would recommend changing him always in the bathroom, and having him try sitting on the potty every time you change him. I personally recommend a small potty chair (we really liked our Bjorn one) so that the child has their feet on the floor. When they're learning to poop, this is essential, because they need to have their feet against something to push. My kids were also afraid of the big toilet (falling in, the flush).
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When you're ready, I'd let him go without underwear completely while you're at home to build body awareness. Moving straight into undies can sometimes confuse a child used to diapers because they're used to letting it go when they feel something against their skin. After he's able to run to the potty and pee without anything on, then I'd add the undies. I would NOT recommend pull-ups. They're glorified diapers (and more expensive). If you have to be out in the early stages of potty training, then I might use them, or for naps. But really, why not just use a diaper for that. I don't think training underwear are necessary, but they do sometimes catch little accidents so you don't have to change a whole outfit.
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My kids trained pee and poop separately. They were both peeing in the potty for a couple of months before they got the poop thing down. I just told them that when they needed to poop, they should tell me, and I'll put a diaper on. Ds did that and resisted pooping in the potty until we made him put his own pull up on, when he decided it was easier just to sit. Dd just waited until we put her nighttime diaper on. When she started going without her nighttime diaper, she pooped in the potty. I don't remember what made her switch, but I'm 95% certain it was her idea.