Am I Being Scammed? (car Trade)

Need to know if I'm being scammed. Traded my truck for a nice ass old mustang (vanilla ice 5.0). 1 have the mustang and the title in my possession and the other guy has my truck and my title. Problem is, he never registered this mustang - he bought it two months ago, signed the "buyer" part of the title, and then held on to it until trading with me. I went into the DMV today and, because he signed the buyer line but isn't the registered owner, they won't let me register it unless he comes in with me to sign some sort of indemnifying affidavit. He has blocked me on Facebook messenger and presumably my phone too (all calls go to voicemail and texts don't say delivered). I dont see what's in it for him to do this but I also don't see why he'd block me.