My son is 3 years old and is just 13 kgs he eats well is active,but doesnt gain weight just a kg in a year should i get him tested for thyroid?

You can: But it can be just his built. How about his height? Your height and his father's too? height is more important than weight in monitoring physical development, both are important though. You mentioned that he is active, so that's reassuring as regards psychomotor development, see his pediatrician,check this link https://www.cdc.gov/growthcharts/data/set1clinical/cj41c021.pdf