Maybe I should not reply, as I haven't got experience with this. My stepkids are much older. I do have a two-year-old, though.
I think that is quite young for the sort of conscious defensiveness you are ascribing to him? But yeah, he doesn't have much time with his dad, and at that age, keeping and preserving the bond with his dad under these limited circumstances should be the top concern, so I guess I would say - let the relationship with you develop slowly, even though it must be frustrating.
Having said that, of course DSS needs to see that you are part of the family and a couple with his dad. Can I ask - what are your times together like? Do DSS and DH spend some time alone as well as some time all together? Do you ever have time with DSS just the two of you?
Do you have much experience with kids yourself? Are you sure that what you are expecting from DSS is age-appropriate? I just know how my DD can be - she's a sweet-tempered kid, but she definitely has just started the bossy two-year-old thing, and has definite ideas about how she wants to play and interact - where you have to sit, who is involved, which cup they can use at her tea party! Obviously I try not to encourage that bossy streak, but I do think it is a natural thing for her to try on, and maybe some of what you are seeing is just that.
One thing that helps a lot with DD is explaining to her in advance how things are going to go....in the morning I talk to her about the day ahead and who she will be spending time with (my DH does the childcare two days a week and I do the others, but we both do a lot of our work from home so sometimes one of us in in the house but not available for her entertainment!), where we will be going that day, if her half-siblings will be at home or at school that day, if any one else is coming to the house. I feel like it lets her know what to expect; obviously she can't hold it all in her mind, but if I have said in the morning "Daddy has to work today", she is less upset later on when she wants him for something and I have to remind her that he's working. If you don't already, you could try to lay out the day for DSS like that - "This morning you and Daddy will go to the park by yourself and play on the slide, then we'll meet Nickname for lunch"...or "After lunch, Nickname is going to help you put on your shoes"....letting him know in advance when you will be part of things and when you won't?