Fun outdoor activities are where it is at for kids that age. He is still a little to young to probably enjoy organized sports, but getting him excited to go biking, camping, hiking, etc with you is the best way to change his mindset.  But, if those aren't things you are excited about he is going to be able to sense that.  So try to find something that is active that you can both enjoy.  When he spends time with you, be sure to enjoy healthy foods together (and more importantly portion control).  Also, be candid with your ex about his diet and lifestyle.  Unless she is a terrible mother, she should care more about his health than her feelings about how the conversation makes her feel.  And point that shit out if she tries to "reeee" at you.