Definitely start getting into shape as a family! Take him to his doctor to get his health checked and to make sure nothing is going on there. If you're having problems sticking to a plan -the whole family can try Weight Watchers! It's just about the only weight loss plan out there that will TRULY teach you all how to eat for the rest of your lives -and how to splurge, work out and be overall healthy.
Not sure about the leg issue and being afraid to ride his bike at almost 10 years old. You may want to talk to him about that and get out as a family to ride bikes.
Make sure he is eating A LOT of protein! He's probably going through some growth spurts, and protein keeps you fuller longer. There's plenty of lean protein to choose from too -meat, chicken, fish, certain cheeses, Greek yogurt, and eggs.
Also, be honest with him -tell him you and his father are making an effort to get in shape because being overweight is awful for your health. Let him know if he can control his weight starting now, he'll be happier and healthier and live longer. Let him know that you two want to be around for awhile and you have realized you must get the weight down! I love what my son's preschool "coach" tells them -eating sugar, fast food, junk and fried stuff makes you slow and sleepy so you miss out on life! It's true too. He never uses the word "fat," but none of the kids want to be "slow" and I've actually had my 4 year old turn down sweets and choose spinach as a side over french fries because he wants to be fast!