Get his ears examined by ENT specialist
and get done Impedance audiometry
to see for the condition of middle part of ear .This test can only be done if there are no tubes in the ears.
As the tubes may come out automatically and then the child has developed the same condition again so get them examined first and then the test.
If there is fluid behind the ears then his might be constantly rubbing his ears because of itching.
As he has a previous history of grommet insertion there are more chances of getting it.