Something that has worked for me before in similar situations is to ask "Do you have a reference recording in mind that would help me understand what type of sound or feel you're going for?" If he says "yeah, check out this Tom Waits record, the thing I really like about the bass is...", then great, go listen to and transcribe that record and then come back to his song and see if you can bring that influence in. Let him know..."hey I checked that record out and was playing with this, what do you think?" It might start the conversation going in a different direction and make you more collaborators. Yeah, you're trying to give him what he wants, but you're making him explain/describe it to you. I've had this work out and I've had it show me that the creative genius I was dealing with didn't know what he wanted or was doing. I had a guy hear me play once and then he called me several times about getting involved in "his music". I showed up with my Fender and he said "Okay this first song...a wait..can you tune your bass (to some made up alternate tuning)?" I said "no, just tell me how the song goes." He said "well, I've made up my own chords and I don't know how to describe them..."
When I got him to play them for me and just transcribed the notes, they were "D7, Amin, etc". He didn't know what he was doing, but he had "invented" some stuff that sounded like music. I was not willing to change for that guy. They are almost never as special as they think they are. A jazz gig I used to play, the guy would shout at us on stage "play it bouncier with loft" and poopie like that. I asked him at a set break, for a reference recording and suggested a few people who he should have heard of (like the guy who wrote the song he was calling) and he said that he didn't really listen to jazz, because he didn't want to be influenced by other people's playing. Well, okay, but all that means is that you can't communicate with other musicians and you're a bandleader who has hired other musicians and wants to dictate what they play and how they play it. Whether he's paying me or not (and he was, quite well), it just doesn't work.
Other's though have said "I really like on x record how the drums and bass lay back and..." and yeah, cool, let's do that.
As for people writing out parts, there's a downside to that too. Piano players are always giving bass lines that go below E below the staff, so not only do I have to read it, but I have to transpose up and octave. Blah. Or big band charts have these "root-5th-root-5th-root-3rd-5th" lines that no bassist would ever play for 23 measures or worse some crazy thing that they never had to finger, but they engineered in Finale, because they were reading George Russell's book or something.
In the end, it's music and it's spending time with people who are either good influences in your life or not. Give it your best shot, give him the benefit of the doubt, see if he can explain what he wants in a more constructive way and then decide if it's worth your time to stay in.
Either way, if you're new to the instrument, start copping some lines from recordings in the genre you would like to be playing in.