Hearing Test

My Lo didn't respond to the bell test during 3mth developmental check and 4 mth follow up check. He could turn his head well and respond to sudden loud sound but seldom look at us when we call him. Any mummies experience this? Also we were referred to NUH for further check up but the earliest appt is in July. Am worried that it will cause developmental delay by then if there is really a prob with my LO's hearing. Any recommendation where else we can go to for a detail check up? TIA!