Ask for Movicol. It softens poo but is also a mild laxative. It's far better than that crap Lactulose.
If he's holding on to it I'd trygiving him things like prune juice. You could freeze it and make it into lollies. Dried prunes in general. i tell my DD they are chocolate and she's very happy with that! Also try chopping them up into his cereal.
Bake some muffins with bran and linseed. Linseed is great stuff.
Try giving him some senacot (sp?) or syrup of figs. Senacot (sp) comes in sachet form too which he can drink as a squash like drink.
A high fibre diet is important but too high and it can have opposite effect.
This is a good sort of daily meal:
Breakfast:
Oats are good so porridge for breakfast, try and chop up some dried prunes. Or weatabix. Brown toast.
Lunch:
Sandwich made with wholemeal bread
bran / linseed muffin
fruit - with skin like plums, nectarines that sort of thing.
Snack : rasins
Dinner:
a stew / casserole type of dinner, brasied beef and veggies or a rich spag bol sauce with homemade tom sauce.
A probiotic yohurt with prune layer at the bottom or more fruit with skins on.
Also try playing around and getting him to lie on his back and circle his legs round pretending to ride a bike.
Make the loo a fun place to be. Sit in there with him, get yourself a low stool. Put posters up, a sticker/reward chart for him. Read or make up stories in there with him. Don't put pressure on him to go but try and stay there for a while and hope he goes.