In a nutshell - I've lived in
It sounds like Mom, Dad and Grandma are also in a power struggle...so it's no wonder son picked it up.
You've gotten good advice on how to try to improve his nutrition, and that if you don't want him to eat it then don't have it in your house. At 13 he can go to the convenience store and buy junk food, and Dad and Grandma might be feeding him junk too. Hopefully if he gets hungry enough when he's in your house he will eat the healthy things that you provide. I have known a couple of kids who were such picky eaters that they were worse then your son, bottom line is that they survived and are healthy adults now (although one of them has expanded his food choices only slightly). So at some point you may decide to stop fighting the food battle, have only nutritious food at home--because that's all you can do.
To me the video game thing isn't about the food, its a control issue. I understand you trying to bribe him with the game--a nicer way to put it is to call it an incentive or carrot on a stick--and its the opposite approach from taking away something he likes. (one of my kids was very challenging especially as a teen so we tried a lot of techniques) The control issues will continue over things like cleaning his room or coming home by curfew or grades in school or whether he decides to start dressing like a goth or gangster. That's just life with a teenager. You'll have to figure out what works with him, incentive or disincentive, and be consistent. Your ex may not be willing to use the same strategies as you, or do any real parenting, so you have to just carry on with your methods. And accept that there are some battles not worth fighting and just let it go...and sometimes when the parent stops letting something be an issue, the kid stops doing it. There is a good board on the P&P channel called Parents of Teens where you can get support and advice on how to deal with the childrearing issues.
I really feel for you, my mother divorced with my dad when I was ten. Co-parenting is so hard, esp if one parent wants to one up the other one or do things which they know will upset the other parent. You are trying to do the right thing, that's the first thing & most important thing. You can't do much when your son is with his father... sadly. Your son is at the age where kids and teens are so selfish that they cannot see anything past me, me, me and the instant gratification. He probably doesn't understand the extra issues that were caused by his grandmother not leaving the cheque so try to leave that out when you discuss the rules of the house with your son.
In your home you are the boss. I would take the game away from him when he's at home with you & doesn't abide by your rules. It won't be easy but being a good mom never is. My mom was hard on me and I appreciate it now. I feel bad that your ex mother in law thought it was wrong for her to take matters into her own hands esp with the child support. That's kinda sick and very malicious. But I'm sure if you get lawyers involved a single letter alone will cost fifteen times more than the fees you've had to pay for the cheque bouncing.
Maybe discuss with your Ex that you'd appreciate it if his mom didn't do things like that when it comes to the support payment. It isn't her job to call those kinds of shots. I know years from now your son will realize you pushing him to study or do the right thing was because you cared. Stay the course, if your husband wants to spoil him you can't control that. Focus on what you can control in your home.
I absolutely agree with you 150%.