Hi.. So sorry to hear about you son.. Well, due to use of steroids your son has developed fungal infection on the tongue known as Oral Thrush leading to white coating on tongue.. You should start applying candid mouth paint on his tongue as it is antifungal medication.. Always wash his mouth thoroughly with water after giving steroid inhaler.. Avoid using antiseptic mouthwash. Maintain a good oral hygiene and feed him with plenty of fluids.. In case if the coating does not resolve even after stopping steroid inhalers consult a Pedodontist and get a clinical examination done.. Hope this helps.. Regards.