19 month old is constipated?

My son has been constipated for about 2 weeks. He has had only little pebble stools except for once when it was very runny. I have talked to the doctor and done everything they said-lots of water, veggies and fruit, less milk, fruit juice and nothing has helped. The doctor has now told me to give him Miralax. Has anyones toddler used it? Did it help?