Mary -
 
Though I'm not anything remotely like a vet, I would think that Merlin might
have an intestinal blockage.  Has he been pooping normally?  He sounds like
he is blocked and is in need of immediate medical attention.  Loosing that
much weight that quickely is a problem.  You should get him to the vet right
away.
 
Let us know what happens.
 
Take Care,
 
Steven
[Posted in FML issue 1681]