He's upset and scared that his present social life will end and that he might not measure up at this school either. There is considerable emotional strain connected to his beginning high school in a boarding school, apart from all that has been familiar to him. I think you and he (don't include grandma in this decision-making process) should sit down and have a few serious discussions about what he wants to do regarding his high school education and why.
You need to be honest with him regarding why you are considering sending him to boarding school. Be aware that you have probably amply communicated your disappointment in him over the years. Perhaps the discussions should include brainstorming ways he could feel more motivated and competent in school. Simply going over what he hasn't done in the past and blaming him for it will be most destructive. What harm would there be with your supporting him and believing in him as he is asking you to do? I know many parents send their kids off to boarding schools with the "Marine-like" notion that the boarding school will "straighten out" their child. That is probably how he's interpreting this. Give me an update on your decision and the contract you two work out.