where do I stand in this little boys life?

So I'm 23, been with my partner 6 years. We had a blip 3 years ago and he got a girl pregnant. We got back together whilst she was pregnant and have since bought our own house and we are engaged to be married. Currently trying for our rainbow baby. 

My partners little boy in now 2. I've been there since the day he was born. We have him every Tuesday and Wednesday, and in these two days, I do most of the 'caring'- I.e bathing, feeding, playing, putting to bed etc. 

We have such a good relationship and I get on with his mum where we text but purely about my partners son. I occasionally have him on my own or pick him up from nursery and take him on days out.

Would you consider me as a step mum to him even though we aren't married. I know legally I am not, but would you present yourself to others as his step mum or partners son to others when they ask our relationship? 

Thanks girls! :)