Im being put on the backburner already with this guy..?

were really good friends for 4 years and always had a thing for eachother but never did something about it till recently..weve been 'talking' for about a month now and we really like eachother. Things got a little heated the other night and we made plans for tonight to hang out. He said he couldnt wait to hang out but now he just rescheduled at the last minute(by the way hes my ex's best friend) n i think they all wana hang out so he asked to reschedule. Which is fine! But i dont want to be a pushover and him be able to walk all over me like my ex did...what do i do so it doesnt turn into that..?