constipation in child and flax seed

Hi
I am looking for advice. My 8 year-old son has been having stomach aches. We realized that he is constipated. His stools seem to be very wide and long. His doctor recommended a cleanse. He wants him to take something over the counter until his poop is like water for 3-4 days, then he wants him to back down on the medicine so that he has soft stools for 2 months.

I wanted to try something that was natural. So we tried ground flax seed and magnesium capsules (600 mg), but I don't think we are using them correctly, because there is no drastic change in his bowel movements. We have been giving him 3 Tablespoons of ground flax seed and 600 mg of Magnesium daily for a couple of months.

I tried the treatment on myself and it worked within 12 hours.

I am thinking I need to up his dosages, but I don't know if it is safe.

Does anyone have any info on this?

Thanks.