Kay - Is this ongoing?
Well maybe start him on nasty sticky lactulose. It draws water into the stool so softens it and helps him pass it.
However it never really worked for me, and isn't a laxative. The best stuff is Movicol.
You can use syrup of figs or senacot - both natural laxatives and are effective.
Make sure he drinks as much as poss. Citrus fruits, fruits with skins such as plums (not apples or bananas though)
Dried fruits esp prunes. Try chopping them up and adding to his cereal.
Wheat and oats good so weatabix and porridge for breakfast and wholemeal toast.
Green leafy veggies good so try sneaking spinch into things like (wholemeal) pasta etc.
When serving the pasta drizzle oil (Udo's Choice oil is good but expensive, if not just olive oil will do). It sticks to poo and helps it's passage out iykwim
Rub his tummy, start at bottom, his right, move up, across and down.
Warm baths.
Do exercises together, lie on your back and do pretend riding a bike.
In dire cicumstances glycerine suppositories will get first lot out.