My 6 Month Old Son Had A Platelet Count Of

My 6 month old son had a platelet count of 39. All of his other results were normal. I obtained the blood test because he is not gaining weight as expected ... he was in the 30th percentile at 4 months and he was in the 10th percentile at his 6 month check up. How concerned should I be? How long should I wait before re-testing? Thanks!