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help my 3yr old from constipation

my three year old son suffers from severe constipation and the doctors say he'll grow out of it.
Our eating habbits are normal.
We eat out once a week, treat for the kids to go to macas.
Fruit and vegies are part of our diet.
Brown bread is eaten at home.
My other two children who are 7 and 8 dont have any problems.
I have tried the parochoc, sepositries, fibre supplements and the only thing that helps him is the microlax which is a liquid that is put into his bottom to help him let go.
Are there any natural remedies out there that i haven't tried?
thank you
worried mum,
Rebecca