When we diaper our kids from birth through 2, 3, 4 yrs of age, we are training them that the diaper is where they're supposed to eliminate, and THEN we wonder why it's so tough to get them to go in the potty when it's time to potty train. But hmm, we actually set ourselves & our toddlers up for this struggle! And P.S., most US-born pediatricians accept and spread the cultural myth of "no sphincter control" - but there's NO scientific study of gentle, early toileting because there's no money from Pampers or Huggies to support any pediatricians who would like to do these types of studies. Huggies and Pampers want your son to be in size 5 diapers because it means more $$ for them!
The plain fact is that all over the world, the majority of babies are using the potty reliably by 18 months - but this doesn't mean they do *everything* by themselves (like take clothes on/off, wipe, etc.). This means that the parents and child work as a team over many, many months in a supportive, practical, no-pressure situation. And very gradually, over time, the child becomes more and more potty independent. No shaming, pressure, negativity. Diapers or some covering may be used, but it is only in case of an accident, NOT as the primary place for pee/poop to go. It works. The proof is in the pudding.
So your 2.5 yr old certainly can use the potty as long as it is offered at ideal times (at what times of day does he usually go?), it is done in a gentle, loving, supportive, no-pressure, non-punitive way. ALL babies, even newborns, can do this - and it's mind blowing when you see it in action!
Toddler/Preschool age is an extremely difficult developmental stage in which changing the rules and introducing the potty are often met with resistance, and that makes total sense! It's why, in our culture, potty training is dreaded by so many parents. ALL babies are able to use the potty - it's the parents who have to decide if they're ready to help their child use the potty.
So, my point is: whatever path you choose, you'll be fine as long as you are respectful to your son. No pressure, no coersion or bribes, no negativity, just love, patience, and support. You can start with your 1 year old daughter AND your 2 1/2 yr old son at the same time, if you want. Just expect that this is a PROCESS in which accidents WILL happen (and that is how they learn, it's not a bad thing!) -- very much like how your babies go from crawling to walking to running. Think of how you helped them become mobile, and support them in that very same way when you potty train them, no matter what age you start.
For more information on gentle potty training, from babies to preschoolers, check out:
http://www.diaperfreebaby.org/Articles/StartingEC.htm
http://www.diaperfreebaby.org/Articles/GettingStartedBobr...
http://www.white-boucke.com/reviews/latestarters.html
http://www.diaperfreebaby.org/PottyingPositions.htm
Happy pottying!
P.S. - Since the introduction of personal washing machines/dryers and disposible diapers, the average potty training age in this country has risen drastically from 18 months old (30-50 yrs ago) to 3 YEARS old (today). Our babies now are no different than they were then - ALL babies are capable of using the potty as long as they have a supportive caregiver to help them get to the potty. And even today, babies all over the world who are under the age of 2 years old can recognize and alert their caregivers when they have to go - again, are babies born in the USA incapable of this? Ridiculous! Our babies can do it to - we just have to relearn this gentle, old time practice that our great, great grandmothers knew how to use. :)