My 4 Year Old Grandson Was Prescribed Miralax For Severe

My 4 year old grandson was prescribed miralax for severe constipation. 2 tsp. am and pm. The end of his tongue is itchy and bothering him, but no swelling, otherwise nothing else. Has had watery stools 5 times today. Should I call the dr? I'm going to stop giving it to him. He had an x-Ray to show how backed up he is.