What can I try next?

I am a 30 yr old mum of 1 boy who is nearly 5yrs old.  He has had nothing to do with his real father and it was just the two of us up until just over a yr when I met someone who we now live with.  My son has started calling him dad (on his own terms he wanted to call him this).  He seemed to like having my partner around but in the last few months he seems to have these little episodes where it seems like his eyes glaze over and he starts screming (very loudly) and kicking and nothing I say or do seems to calm the situation I have tried ignoring him when this happens, I've tried smacking him on the bottom (which I hate doing) and even staying calm doesn't work it's like something just clicks on and then off again when he's ready this can last for 5 mintues or an hour.  Do you have any suggestions on what this may be or what I can try so as this doesn't happen any more.  This seems to happen only when he doesn't get his own way.

[Encephalomalcia]

He may feel like he is competing for attention from you. He may be frustrated if he feels that your significant other is taking attention away from him.  Make sure that he is getting a lot of attention from you.  Spanking will only make things worse.  He is jealous.  

[nicci464]

Thank you I understand what your saying.  I do feel like he is doing this so I do make a special effort to do things with just him and I and this still doesn't seem to make a difference any other suggestion??