He is always pushing me to turn the treble way up and the bass way down.
In the words of the immortal Aussie legend, Darryl Kerrigan - "Tell him to get stuffed!"
There's so many things wrong (figuratively and literally) with what your guitard seems to be saying that I won't even address them. Suffice to say you should just find a nice way of telling him to stick it where it fits and let you get on with doing your job. Either that or give him a taste of his own medicine - "Hey man, your distortion pedal has too much X and not enough Y..." etc.
Now, a passive volume pedal IS just a pot - IF it's connected DIRECTLY to a bass it essentially behaves
exactly like a volume knob. In fact, it could be argued that this scenario is even worse for your tone because now the impedance of the volume knob and the pedal are in parallel, and hence both contributing to any tone loss, real or perceived.
However, putting a volume pedal AFTER an active device (on-board preamp, effect pedal, amplifier preamp etc etc) OR using an ACTIVE volume pedal is a whole different box of fish.
It all comes down to buffering. The short answer is, if you have an active bass then there's absolutely no need to worry about where you put your volume pedal. However, you should make sure you use a volume pedal designed for active basses (generally lower impedance/resistance.)
If you have a passive bass, then where the volume pedal is in the chain can become important. To prevent any potential tone loss, place it AFTER at least one buffered effect pedal (or other active device).
A further caveat stems from the utterly ridiculous "technology" known as true bypass - no buffering means TB pedals are not suited to use for the above purpose.