Take a deep breath, and get some perspective. He is almost seven. That is not an unusual age for a second grader - he is probably within a few months of age of many of the kids in his new class. The bad experience he just had was with kids up to three or four years older than he is. The difference between being with second graders and being with fourth graders is a big difference. So there is no reason to think that this won't be much better for him. Instead of looking at this as him being placed with older kids, you could easily and accurately look at it as being placed with kids more his own age.
It sounds like working on expressing his feelings through words before they become so intense that he has to express them through actions is probably going to be an area where you and the teacher will need to give him some support and direct instruction. You might also want to give some thought to teaching him self-soothing or calming strategies. If you have an IEP, try to find a way to get behavioral supports that you think would be helpful written in. If he gets overwhelmed in new situations easily, there may be some sensory integration/OEs going on. Has he ever been assessed for this sort of thing?
I wouldn't make long-term projections about his ability to handle being with older kids in general based on a few days at a summer camp. A lot depends on the particular children in a given group, and on the atmosphere created by the adults in charge. If he got to the point of completely losing it, it sounds like the person in charge wasn't doing a very good job of paying attention to the group dynamics.
I would present it to your child with the attitude that this a great thing and that school is going to be better this year than last year. If he is specifically worried about the other children being older, I would point out that he is going to be seven during the year, just like the kids in his new class are seven.
