All the usual advice really.
Approach RBL, RAFA and the museum at Elvington re a visit to Friday 13th.
RAFA or RBL can supply a flag to drape the coffin.
Is there a squadron association for 158? If so involve/advise them.
Any links with the local ATC squadron?
Do you want/does he want to donate any memorabilia to a museum or library.
Is he still 'with it' - no disrespect intended, but his awareness might make a difference as to how you handle the situation.
What have the medics said regarding timescales, palliative treatment and the progress of the illness and the likely side effects of treatment. by this, I mean he may have been diagnosed with lung cancer but the symptoms being displayed might be reasonably masked at present.
Has he got a bucket list, as this might make his life fairly fulfilling. If you can persuade him, has he recorded his experiences for posterity.
Just some thoughts.
Old Duffer