4 year old constipated

Help my 4 year old has suffered on going constipation really from being weaned onto food. He had a health diet and I try and increase fibre in his diet by buying the bread etc with extra fibre. He has never been great at drinking I think that's the problem. I really try to get as many fluids into him as I can. I have been giving him a tiny dose of lactulose 2.5mls daily to help which is working but sometimes he's still going 3 or 4 days between bowel movements. His bowel movements can be huge and he can have staining on underwear so definitley constipated. Was thinking trying little cartons of apple or orange juice even. Which would be best?