Not to give them immunity against weapons that weren't made on silver...but:
SUGGESTION OPTION 1:
What about give them a 5 DR against non silver weapons? (and not only to the werewolves pc, but also to the npc weres...
Then, encode the current silver weapons the npc sell and the masterly damask weapons pcs can craft (and that currently now count with silver in their alloy), to ignore that 5 DR.
5 DR is not too much, but enough like to give again to the silver a meaning as a nice feature in the Werewolves theme that many (I think) liked.
SUGGESTION OPTION 2:
Another option (easier) may be give them 5/+3 DR... (as the silver weapons npc sell count as +3 against werewolves and as masterly damask is +3 to hit, that is equal as to say you need silver or a powerful enchanted weapon to bound them better)
ADDITIONAL SUGGESTION OPTION 3:
The DR may be scalating in a small degree, starting at level 3 player as 1 DR and increasing with each 3 or five character levels, so if each 3 levels, they may get up to 10 DR in level 30... or if each five levels, 6 DR at 30, which is not an OP thing at all, but gives them the resilience against standard weapons that only silver or a powerful enchanted weapon (+3 or better) may penetrate.
(Because is sad that common sewer rats may slay a werewolf who was a fighter level 5
)Thanks for your time and great work.