I don't think the dentist doesn't know what to do; I think he's
pointing out the dilemma with this tooth, and in a wishy-washy way
telling you there is a risk to the tooth whatever is done.
    The filling does not of course stick to the gum; it has to stay in the
tooth.  If the filling is significantly below the gumline it will be a
continued irritant to the gum and perhaps encourage further recession.
    Having the gum tissue lowered (sometimes called crown lengthening,
sometimes gingivectomy) may be an option, but the risks here are
increased thermal and touch sensitivity (if the pulp is vital), and
further root exposure.  Oral hygiene and caries control are vital here,
otherwise decay will just resume again after the filling.
    Sometimes some increased attachment of the gum can be achieved, or
greater resistance to further recession by what is called mucogingival
surgery.  This is something to discuss with a periodontist.  However,
you are now talking about a fair amount of dentistry, so if the tooth
isn't otherwise in decent restorative and periodontal health, you have
to consider whether or not the time and expense are worth it if the
tooth may not hold up.

Good luck,
Steve