Elevated ALT in 14 months

My son had his 1 year checkup 2 months ago.  His ALT was at 51.  His doctor suggested that we keep an eye on the ALT.  Since then my son had done the test 2 more times and the level is going up (59 and 92 respectively).  Other test results were within the normal ranges (Alkaline Phosphatase [209], Calcium [2.51], GGT [20], Phosphate [1.64], Hemoglobin [112], Hematocrit [0.33], RBC [4.0], Mcv [82], Mchc [343], Rdw [12.6] Platelet count [404], wbc [8.2], Neutropils [0.9], Lymphocytes [5.1], Monocytes [0.6], Eosinophils [1.5], Basophils [0.2], Calcium [2.61], Creatinine [17], urea [2.7], ferritin [24], Iron [11], Tibc [52], Transferrin Saturation [0.21], Protein total [59], Albumin [38]).  Also Anti-HAV (IGM) is negative and Anti-HBS is 112.1.

My doctor said he doesn't know what is causing the ALT to go up.  He suggested testing the ALT again in a month and also test for Hep A and C.
Questions:
I am giving Iron-supplement to my son everyday, will that cause elevated ALT level?

I haven't been fasting my son for 12 hours before the test, will that cause false test results?

Can you describe the most common causes for ALT to slowly go up?  

I also read that ALT level can be twice as high in infant.  Is there adjustment for 14 months old?

Thanks for your response.