Anyone else's kid refuse to go to the other parent's house?

My 13yo son has been refusing to come to my place for the past 3 weeks. He says he's "too busy" with school projects and friends but I think it's because my ex has been badmouthing me to him. Or I'm a shitty dad. Honestly could be either at this point. I don't want to force him, but I also feel like I'm losing my relationship with my kid. He used to love our time together. Did anyone else deal with a teenager pulling away like this? Should I just give him space or try to rebuild?