K.H.
L.,
OK- don't be scared. I'm certainly not an expert but I have used it in the past. Why not try it out on an old rag first. That way you'll get a sense of how it works... AND if the pants are already greyish than you have nothing to lose!
For his next pair of pants you could try Tide Coldwater or Woolike for Darks. Maybe those will help keep the color longer.
Good luck- I totally get... I've shrunk SO many of my husbands things!
K.