Continuing to send him away, without either parent, for the entire summer is setting him up for a couple of things: He will resent you, and the relatives who are caring for him; he will be angry at being sent away; and he will start to wonder if he has done something wrong and is being punished by being sent away.
YOU know none of these is true -- the relatives love him, he shouldn't be angry, and he's done nothing at all wrong. But he is still very young, and he will hear any talk about "going to granddad's next summer" not as some nice adventure but as being sent away. And kids look for reasons for things, and it would be typical for a child to turn this on himself and start to think, "I must not be a great son/must have done something wrong for mom and dad to do without me all summer."
Again you know that's not the case, but you need to try very hard to see this as HE sees it, in his seven-year-old mind. You can reassure him all you want; you can talk and talk about why your jobs that put food on the table mean he needs to go; you can talk positively forever about how much fun he'll have at grandad's; but it will not matter. He will still see this through his own lens because he is seven, and adult reassurances will not stick. That is not wrong of him; it's normal. Don't let anyone else make him feel guilty about being upset over this. (Just in case -- If your husband is a guy who would say, "Man up, stop crying," you need to be your son's advocate and not let dad dismiss his son's very real feelings.).
I am not saying this to make you feel guilty -- please, please understand that. But I am trying to get you to see that his view is not, at age seven, or even this summer at eight, going to change. He wants you and his dad and feels insecure -- event with relatives. You need to look through his eyes and his emotions, not adult ones.
You live in a very large metropolitan area. Most such areas in the country have many, many summer programs aimed at families like yours where kids MUST have summer care. There are half- and full-day camps, some of them inexpensive (try YMCAs, church programs, programs sponsored by the school system for less expensive options); summer day care at paid providers' homes; some preschools use their space in summer for day care for school-aged kids; local parks and recreation departments run camps and classes designed to keep kids busy because parents are working. There are many options in an area like yours. I would suggest that you start, today, looking into those, so you can book things now -- these programs do fill up. But I would not send him so far away for the entire summer again.
Anotherh option: Can granddad come to you for even a few weeks, if not the whole summer? If the aunt and two cousins live in your area, can the aunt provide some care coverage part of the time?
In other words, put together whatever you can -- your and your husband's vacation time; family care; programs and camps and classes --to keep him at home. And don't schedule every second of the summer; try to have weekends just for the three of you in your own family. He will feel more secure at summer's end.
If paying for camps and classes and day care is an issue, check into sliding scale fees or "scholarships" that some offer, especially those that are provided by the school system, county or city government, YMCAs and YWCAs, and churches. (Our school system has a full-day summer program that takes place at school facilities and it's very inexpensive compared to for-profit camps -- I would check with your school first and foremost before their program, if there is one, fills up.)