Toddler witholding poop?

If related to . starting toilet training,put him back in diapers till his doctor gets him "cleaned out" & on Miralax (polyethylene glycol) with soft stools at least every other day for ~ 2 months. If toilet trained, use a stool like the " Little Looster" to prop his feet so his knees are flexed > 90 degrees. Put him on the toilet for 15 minutes, 15 minutes after supper. Let him blow bubbles, color, etc. Do attend to it ASAP.
Habit constipation. Is a common problem in toddlers. See your pediatrician for treatment. If it goes on longer, it is harder to treat.