How to toughen up my nine-year-old?

He is an only child and hasn't been around a lot of boys and I admit, I babied him a little as a toddler but now he won't play sports, he won't do flips on his trampoline, he won't even jump off anything higher than 2 feet. I just don't want other kids to have a reason to make fun of him. I have tried to toughen him up but I'm afraid I come off more as critical of him instead of encouraging and I don't want to make him feel bad about himself. My husband refuses to put him into karate or anything like that so idk what to do. I took away his xbox and iPad and told him he needs to play outside instead but idk if that will help at all? :\

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