H.W.
I love this site:
http://www.betterkidcare.psu.edu/AngelUnits/OneHour/Toile...
This will take you step by step through assessing your son's readiness.
It's written for early childhood ed students and is probably the most concise and effective method for toilet learning/training I have seen in my nearly 20 years of working in childcare.
It's very, very wise of you to be thoughtful about approaching this next step with your son. Sometimes, we initiate it because we see some 'signs' and because *we* are ready for them to be using the toilet. What I have discovered over the years is that when children will get it when *they're* ready. Families that start before their child is ready often end up with more frustrations, taking as much time (or longer) than if they'd just waited until their child had all the skills they need.
Consider it like this: if you were to take your child skiing, you wouldn't push them down the hill with just their boots on. You'd make sure they had the skis, too. Likewise, having this set of skills this site suggests will give them all the intellectual, language and gross-motor readiness they need for taking the next steps toward independence and communicating this with you.
For what it's worth, when my son was nearly three, we tried underpants for day or two. It was really apparent he wasn't ready. We waited just a few extra months, then offered him cotton diapers or training pants, and wow! what a difference. He was really ready the second time, and it's been incredibly easy since. 3 months after we started that, he was staying dry all night. I know this isn't the case for every kid, but I do think waiting a while gave him time to develop the skills he needed to succeed, and because it wasn't a struggle, this has given him confidence to continue doing well.
Lastly, one of the largest struggles children have when learning how to respond to their body's cues is leaving off play to use the bathroom. Two suggestions here:
A. Don't ask them if they 'have to go', instead, invite the child to the bathroom every hour and half or so. (if they don't go right away, just repeat the request a short time later) If we ask, it gives them an opportunity to say "no" (age appropriate expression of autonomy too!), which then has the potential to become a power struggle. This is a time when you, the parent, know best, and your confidence in guidance "It's time to go use the potty" will help immensely.
B. Find ways to 'save' what they are playing with. At my preschool, when kids worry that someone might move what they are playing with/take it, I either put toys up or place a scarf over them, meaning that we are saving them for someone. This, too, honors and respects their emotional needs when using the toilet-- the need for their work(play) to be secure. I mention this as your son has siblings, and this may sometimes be a reason he'll be reluctant to leave off playing. Teaching our children to respect each other's work is helpful in this regard.
Good luck!