this is a tough issue. my sis is bipolar and refused medications. it drove my mom nuts and still does. about 15 yrs ago, when she was 40 yrs old, she had to be committed against her will.
she now complies with medications after being hospitalized sevral times over the yrs. she hated the psychiatric ward.
my mom still frets over her and calls daily and asks her if she took her medications. that makes my sis mad.
my sis is also over 300 pds, diabetic and Chronic Obstructive Pulmonary Disease (COPD) on 24/7 oxygen. she knows what she needs to do to feel better and lose weight. no one can force her to comply. she eats what she wants. sometimes this causes her to have to go to the hospital for her diabetes.
she is 56 now. she is in control of her own life. we fear she will die young because she will not let anyone monitor her situation. as they get older, they get more defiant and do not want control.
seems you are doing all you can. if your son is educated about his medical issues and told what he should do, what more can you do? you cannot control every bite he eats.
Can you over to join a gym or walking program and do it with him?