Hi, my ds is 4 too and has always suffered with constipation. Lactulose isn't really that great (it's cheap though). My ds now has at least 2 sachets of movicol daily, I put these in smoothie drinks, the only thing he doesn't notice me hiding things in!! I think I have him under control now, fingers crossed as I've had him in the hospital 3 times this year, the constipation was stopping him from passing urine!! Hope you get him sorted its awful for them.
My 5 year old suffers from severe constipation with overflow. We have huge food issues with him that lead to this and to get him to have Movicol is now impossible. However, we have now been prescribed Resource Opti-Fibre. You cannot smell or taste it at all, and believe me he would be able to. The only thing I'd say is that you may want to put it in a water bottle of some sort (Ben 10 for example) so that you cannot see it because it does look slightly cloudy. It doesn't change the consistency and my son has never complaioned. The dietician has given us everything available for his other nutritional needs and he accepts nothing so you can grasp how good this stuff is. Also I got in touch with out community nursing team and they were fantastically helpful, although they do go down the movicol route most commonly but when you can't this seems to be the best stuff. Things are easing for us, not perfect but better.