The fish itself isn't important, but what it represents is important. Similarly, the decision itself of whether he keeps it or whether you take it back isn't important, but your process around this is important. So I think you are right, objects might not hold the same significance for him as for you (and of course that is fine), but what should be significant to him is your process. He should be able to pay attention to what is happening for you and help you explore what is happening - which might not even involve reassurance. However, hopefully it would involve him remaining congruent whilst also caring about your responses, meanings, experiences, feelings, associations, fears, and all of the goings on in your process. This seems like relational work and maybe that's why he isn't serving you or the work very effectively.

And just to say about the idea of you needing to make yourself small to be acceptable, whilst I can certainly read that in some of what you say (not wanting to repeatedly discuss it, giving a small object etc), I also see how big you are being. You are taking how you feel seriously and you know that this matters. You want to talk about it here, you are driven to speak to him about it, you are having big feelings about it. I think this is part of your agency and good for you for taking up space even at his inconvenience and your discomfort.