Is the wee guy actually constipated at all?
or is he holding poo for the sake of it?
I would say to you, if he is at least letting his poo out when he is on the Lactulose, just keep the wee man on it.
Lactulose is so safe, as in you can use it for a long long long time...my daughter has been on it daily for going on a year...no worries at all of becoming dependant on it, all it does is merely keep the poo soft.
When kids hold their poo it more often than not goes hard, then they get constipated, then they cant poo, and when the cycle of chronic constipation starts it can turn to a condition called encopresis.
Please note, I said CAN turn to encopresis, this is not to say all kids affected by constipation will get this, but some will.
Encopresis is caused when the rectum is so full of poo that is no longer coming out, that the rectum stretches and stretches, thus the kids no longer feel the need to do poo,.. and then they start getting skiddies in their undies...that they are usually unaware of and totally unable to control.
Then you end up with a very nasty problem on your hands, quite a few of us are in that boat, but we all try our hardest to educate other parents about the problems of long term constipation.
Hope I have helped you feel less concerned about the Lactulose.