We use Movicol Junior for this reason, we were told we could use Half but Junior is flavour free. Half has more sodium content or something on comparison, I cant remember exactly now, but they were comparable. Half has flavour, Junior doesn't apparently, Ive never tried either . Ive never had issue getting him to drink junior, and he will drink it straight with water.

Previously we've used both parachoc and lactulose to no effect.