1 - Silencers do not work on companions.
2 - to make companions use non-standard ammo you have to
remove all other types of ammo from their inventory. If they have multiple types in their inventory, they will always use up the standard ammo first. Thus you can't stock them with multiple types of ammo and expect them to use the best. However I'm not sure if they'll use
all types of non-standard ammo. I know they'll use AP rounds and some have mentioned them using handloaded/crafted ammo but I don't craft so can't confirm if it's true for all types.
Edit:
I have over 600 standard .308s to give him too.
Companions will go through ammo MUCH faster than you do. Just something to keep in mind.