ADVICE PLEASE

My SO is coming to visit on Wednesday. My mom texted me saying she and my dad want to take me out to dinner to celebrate my acceptance to a conference that same night. They don't know my SO is visiting because they usually over analyze his actions and make mean comments about him. For example they say he's "the kid who won't shut up" and are convinced he is manipulating me because I'll stop whatever I'm doing to read a chapter he's written. I really enjoy reading his stuff and he always tells me to finish my work before I read anything. 
Anyway, I asked my mom if we could reschedule the celebration because I have a lot of work I have to do before I leave Thursday morning (this is actually true). She replied it was no problem, she and my dad can just hang out in my dorm room while I do my homework. She basically just elbowed her way into my room despite the fact that I'm busy. I saw her last Saturday and she already wants to see me again. I don't want to tell my SO to scram for 4 hours, but my parents would freak if they saw him. What would you do?