To let go, or not to let go...

THAT is definitely the question. Older son "J" came home with his progress report today. NOT GOOD. Not even close. First things first, this morning I got a call from his teacher asking if I've seen and signed his take home folder for the past 3 weeks. When I said no, she informed me that he has signed my name for the past 3 weeks attempting to pass it off as my signature. She also informed me that he has not been doing his assignments...6 to be exact. When he got home, I almost blew up! I mean fifth grade and three F's and two D's? Come on. So here's the real problem. He and his brother "C" are supposed to be going to Colorado this friday to spend thanksgiving with their dad. My husband doesn't think I should let him go. And I pretty much feel the same way, but I had already mentioned to him that he still could go. Can I re-nig something like that? I don't know what to do. What am I supposed to say?