helen, have you tried to talk to your dp about it so that he actually knows how upset you are when he leaves you?
how are you when you go out with him? are you sociable or quiet?
what ever the reason, i dont think its on, its terrible behaviour and you shouldnt have to put up with it if it makes you sad which it obviously is.
i think you really need to tell him how you feel, and try to find out why he does this to you, ask him if its anything you do that makes him want to go alone
how long have you two been together? sometimes i think men dont want to go everywhere with there partners because they feel like they are commiting themselves too much, and want there independence
me and dh always go out together with his works doo, but when i go out with my mates he dont come with me, which suits me fine because he is very quiet when sober and im forever worried that he isnt enjoying himself thus making me not being able to let my hair down completly iyswim
however if he wanted to come, i would never ever say no...so imo its not on what your dp is doing
good luck