Going to chime in here on this one.
No, no and NO!!!
They are of course going to be curious about their father and the rest of his family, however, they are kids. You are the adult and parent, who must make the final safe and sane decisions regarding the choices that are offered in life. Make the safe choice!! Yep, they may be disappointed and/or mad, but they will get over it. And if they push for an answer, as teens do, I would explain to them that they are your whole life and you love them very much and could not live with it if they had a bad experience, or worse, in allowing them to go at this time, and that something ELSE can be arranged in the future- maybe next summer, where a bit better arrangements can be made. They are old enough to understand what you have been through and why you are fearful.
Also, your boys don't know any of these people from Adam. I would not allow them to go for a full week, into the care of someone they dont know, that far from home, NO WAY! His parents MAY not appear to be abusive, but those are still his parents too, and if he lies to them and takes them for a couple of days to 'spend time with them' - you would have no idea. And I am sure he is going to want to discuss you with them.
Even when they are old enough, I would set up perhaps a WEEKEND trip, meet half way, and be there handy- supervised visits only, until you yourself can see what is going on. If he is this type of person, is this what you want your boys exposed to?! And I can tell you that abusers like that do not change and they cannot be trusted!!
Why have the grandparents, if they are so 'ok', never been involved in the boy's life??
I am sure, when you change your mind, he will once again tell you how low and crappy you are- they are good at that- just consider the source and let it roll off now. You have a loving family with two great boys. Keep it that way!! Trust your gut and DONT DO THIS!!!!