lactulose (works by allowing liquid into poo and making it softer)
Not sure what age you can start in from but senacot (natuaral plant laxative available in chemist)
if he hasn't been for ages and you feel able - now andf then only, a suppository for children, 2g glycerine ones.
crush linseeds and put them over his cereal.
try giving him porridge or failing that weatabix or shreddies
cook his veg, like some sweet potato or wahtever in freshly squeezed oj
prune juice (to drink and try making them into lollies)
raisins
fruits such as watermelon as well as oranges etc.
massage his tummy.
stimulate his rectum with a lubricated finger.
Sit him on potty and rub his tummy while sitting behind him and maybe do this while watching the tv or something so he's relaxed and not bored etc.
lay him on his back and move his legs round, although riding a bike
bowel essence fromhealth food shops (tinctures of flowers that promote a healthy bowel)
use oilive oil in his cooking
you can by oils in health food shops that you can pour over his food, like spag bol etc