I said i'd go out with him cause i was afraid to hurt his feelings?

i used to date this guy, but then i broke up with him. i'm now seeing this other guy, and i have been for about 5 months. my ex wanted to hang out for a while now, but he doesn't know i'm seeing someone else. well i've been ignoring his texts and making up excuses not to see him but today in the parking lot he comes up and like bombards me and asked me out. i couldn't say no to his face, i didn 't want to hurt his feelings. now i'm supposed to go out with him this weekend. should i just go out with him and after let him know that i can hang with him, but not date him? or what should i day? i just really don't want to hurt him, but i would like to remain friends. i didn't want to tell him im seeing someone else but i think i might have to. advice please!