Late to the party as usual here. But this is the advice we got from one of the top docs in the country who has a clinic devoted just to treatment of severe, unresponsive constipation.
To restore regularity, use Miralax along with the usual dietary measures. If Miralax causes leakage add a product like Benefiber which will hold everything together without making it hard.
If this is a recurrent problem then he may need to use Miralax for several months on a daily basis so his colon and rectum can return to normal function after being stretched out by the accumulation of stool. (Sorry too much information I know but us parents of kids with constipation and Crohn's know ALL there is to know about poop and are not shy about proving it.
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Once regularity has returned, have at least one 8 oz serving of one of the following every day:
Apricot Nectar
Pear Nectar
Prune Juice
Apple Juice
AND at least one serving (prepared any way you like) of:
Pears with the skin on
Cherries
Apricots
Avoid constipation promoting foods like:
Dried fruit
Applesauce (yes, it's constipating but apple juice isn't. Go figure)
Rice
Cooked carrots
Bananas
Hope things already got going and he's a happy camper now.