Limping 2 year old

hi, can anyone help me my son has had a limp now for 4 weeks its very noticable, we have seen a peadotrition and had xrays of his hips and pelivs and various blood tests all fine, hes fine in himself but was very very late in reaching all his milstones, and cant climb stairs or jump yet,he still does everything he always did but just does it with a limp, he dosent seem in any pain, our doctor isnt very helpful and i am worried he is missing somthing is there a chance this could be anything serious what shall i do from here?

Many many thanks x