Toddler not pooping

Hi

my 18mth old son has great difficulty going for a poop without it hurting him,  we've tried liquid lactose, fig syrup & brown sugar in orange juice.

took him to the doc's and he's now on Movicol (4 sachets a day),  this works until we reduce the sachet doseage then he just stops going again or its a very hard lump which is very painful to pass.

he drinks plenty of water & fresh juice, eats fresh veg but only a very little fruit.