Have you checked these resources:
 
http://www.miamiferret.org/fhc
       including
http://www.miamiferret.org/fhc/melatonin.htm
http://www.miamiferret.org/fhc/adrenal.htm
 
http://fhl.sonic-ferret.org
       including
http://fhl.sonic-weasel.org/browse.php?msg=YG1770
http://fhl.sonic-weasel.org/browse.php?msg=YG1780
http://fhl.sonic-weasel.org/browse.php?msg=YG2381
http://fhl.sonic-weasel.org/browse.php?msg=YG1741
http://fhl.sonic-weasel.org/browse.php?msg=YG533
 
http://www.smartgroups.com/vault/ferrethealth/ReferenceShelf?
viewtype=Detailed&shownum=20&sort=type&dir=asc&startrow=1
 
Notice DATES because information changes over time!
 
Personally, I consider it a mistake to not use the proven approaches
which exist.  Sorry, but adrenal growths so greatly impact on life, and
there certainly are approaches with a greatly proven track record in
testing: Lupron, Suprelorin (less tested but three studies exist and
reports from those in Australia and Europe trying it have been promising,
too), melatonin implants.  Propecia.Proscar, etc.  Reports on saw
palmetto in ferrets appear to be few and varied.  The biggest problem
with the grouping you mention is that only melatonin tackles some of
the source of the problem, and there are stronger approaches with more
promising track records.
 
BTW, I am personally curious why he is "possibly a Marshalls ferret".
The dark dotted ear and toe tats for MFs are distinctive if the ear isn't
missing or messed up.  If what you have is a pink dotting them he is
Triple F.  If there is a line combo which approximates an H then he is
Hagan.  If the dotting is a mess or is fading badly, or was temporary
he might be one of the fake MFs from backyard breeders who show up from
time to time.
 
Ferrets from any source can get adrenal growths, and some whole ferrets
get them, too, though the rate is apparently decently higher (vet
observations in multiple resources available to all) among sterilized
ones no matter what age they were sterilized.  (Sterilizing at 6 months
appears to be in the noise when on-set ages are looked at because the
deviation is greater than 4 or 5 months), a promising hypothesis which
doesn't seem to have panned out, as certainly can happen to any
hypothesis.)
[Posted in FML issue 4832]