Dealing with boyfriends other child??

I have nothing against his son but my boyfriend no longer makes time for me anymore. I don't want to seem like a butthole but everytime me and him has made plans or just simply is going to spend QT with eachother they always get cancelled. Now I do agree that it's his child and he has a responsibility but I feel like between work and his son he's let me slip away from him. I've been very patient and understanding since we've got together 4 months ago but it's getting a little frustrating. The last time I tried to mention that I don't feel like he cares about me because he always has something other to do rather than spend time with me.. He completely snapped! I don't want him to put me before his son nor am I trying to take his place but me being pregnant and stressed all the time leaves me feeling left out. How do I stress it to him without him getting upset??