...I'll say, Jess! 

Sunraysia prune juice is the stuff you're thinking of.  freezing it in icecubes helps 'cause that's a heck of a lot of yukky yukky juice to get through before it goes off, and it goes off in about a week or so. 

Glad it worked for ya, Eva.  If he gets constipated again (thanks to Ella's prob I have a ton of experience with this issue!), as a rule of thumb, prune juice is gentler on the system than Kiwi, the juice of either fruit is best but if they don't help it's time to move on to the puree'd prunes (cook em up slowly first to fatten them up), then pureed kiwi (raw - choose gold kiwi not green as it's less likely to cause reactions).  Avoid banana, and give extra water - in a syringe if need be!  Keep up the good work with massage, and remember to rub his tummy in the direction that his lower bowel works in (hand to the left-hand side - your left as you  face him, that is, of his belly button, over the top of it, and down the other side), and exercising his legs.  And if need be, there's always Coloxyl or Lactulose from the chemist - Lactulose is made from fruit sugars so works in a similar way to the brown sugar.  If it gets seriously bad, a Doc can prescribe an enema that is much easier and simpler to use than it sounds! 

And BTW - my plunket lady recommended the brown sugar as being worth a try too.