How can a child who "has grown up too fast" be helped to reverse that and be more childlike and less anxious?

first you need to find him an outlet for his frustrations. drawing, going to the park, playing sports. when people get stressed taking up a hobby helps to reduce that stress. or maybe teach him to play a musical instrument. that requires concentration as well as is self rewarding. one day he can't play a single not, then with some time and work he can play notes... then he can put them together to make up a song... etc.