I do agree that you need to let go more and stop calling at all when son is with his dad.
But there is one red flag to me. if dad and stepmom truly will not ensure that your son is doing his homework on weeks son is with them, I would think that is a very real reason for some changes. Hasn't his teacher or the school counselor been in touch with you or with dad to say that your son is in academic trouble because he doesn't do assignments? Or is he doing them on his own the best he can? (If so, praise him a lot for that, no matter what the grades turn out to be!)
If your son is actually turning up at school on dad's weeks with nothing at all done, and if that means that when you have your son, you aren't aware of things that were assigned during dad's week because dad does not communicate with you about school -- that is going to be a big, big problem very soon. Does dad have the attitude that "It's the kid's homework and he can do it or not, and take the consequences at school, and it's not my job to oversee homework"? That might not hurt a first- or second-grader much but it will soon begin to show up when your son can't understand things in school because 50 percent of the time, he is not following up with the homework and adult supervision that are needed. Eventually, homework advances what the kids learn and they need to do it to understand what happens in class. That's not the case yet at seven but it will be soon.
And kids need to "learn how to learn" -- abandoning a kid to "Get your homework done or not, whatever" creates a kid who doesn't know how to set priorities because the adults didn't model it for him. Kids of seven don't have a clue what to do if dad or mom says "Go do your homework." If he's not guided now (and guidance never means doing it for him, but knowing what he has been assigned and helping him figure out what to do first, etc.), he will not have study stills in place by middle school and that is too late.
Do you ensure that you know what son's homework is when he's with you? Do you have relationships with his teachers so they know you? Do you keep up with his progress between report cards? Do you sit near him as he works, not to do it for him, but to help him feel motivated and to answer questions when he feels overwhelmed? If so -- good for you. If not -- start doing that.
I would ask to talk to his teacher(s) and get an in-depth review of how he's doing overall. He should not have a ton of homework at his age, but what he does have should be taken seriously and done promptly no matter where he is on any given week. Do not, not, not go into a discussion with a teacher and start bashing your ex or his wife -- that will make the teachers stop taking you seriously and dismiss you as an over-emotional mom out to hurt her ex, not help her son. Just say that your son is with you only every other week, and you want to ensure that things are consistent with his schoolwork. Possibly a very understanding teacher will e-mail you each week with the assignments so when you get your son back after a week at dad's you can check what has been done....
Does the ex take your son to any school events? That's not essential but if it keeps happening over the years, your son will eventually realize dad's weeks are ones where he misses out on a LOT. He will not progress in school, won't progress in any sport or other activity he tries, will miss Boy Scout meetings he wants to attend, whatever.
Unfortunately, my friend could not force her ex to take their son to anything, and dad would not "because it's MY time with him," but once son was old enough to say, "Dad, I need to get to my game this Saturday on your weekend" and "Dad, I need to be at school one night for this event" -- then dad started doing those things. When the son was younger dad mostly kept son to himself to show mom that dad didn't have to do a darned thing that "MOM wanted him to do." The son had to get old enough to assert himself and say HE wanted to do X or Y before dad would take him to do stuff. Frankly you may have to wait it out for your son to be old enough to tell dad, "I want to do X" before dad finally gets involved that way.
Please see a counselor and please get one for your son too. If he is hating dad's house your son might benefit from talking to a counselor who works with kids whose parents are divorced.
One last thing. Your son will pick up on your anger even if you never say a word to him or never once badmouth dad or the stepmom. Kids are very good at picking up on adult emotions. He knows, on some level, how much you hate stepmom and resent dad. You totally have a right to your emotions for yourself, but please get counseling ASAP to find ways to protect your son from your emotions. He will continue to fight going to dad's as long as he knows you are seething with anger (however justified that anger might be) every time he leaves. Don't ever let him see your upset or anger in the name of "being honest with him about how awful his dad and stepmom are" or anything like that. He is not old enough to process that kind of "honesty." Get a counselor to help with techniques for calming yourself and stepping way back from contact when your son is with dad.