P.M.
The best resource I've found for potty-training alternatives and general information is http://www.parentingscience.com/potty-training-tips.html.
There are a number of different training approaches/ages, any of which may be more appropriate for different mothers and different children. And both physical and emotional readiness is essential for the most common approach used today in this culture. Read about just about any potty training question in helpful detail.
Your son's inability to poop yet is not unusual – pee training is often successful before the child is able to connect the dots on pooping. And I'm glad you're watching out for withholding on the bowel movements, which can result in bigger difficulties down the line.
There's another resource which may eventually be useful for poop withholding, but I suspect your son is still too young to get it conceptually. You could check it out, however, and see if this system might just work naturally into your at-home routines: http://www.rogerknapp.com/medical/pottytrainingrefusals.htm
Best of luck to you both!