Possibly, but still, he's underaged - that picture of him looks more like the twelve-year old than the college-aged Dick Grayson to me, anyway. As such, he's still Batman's ward, and that makes it HIS decision, not Superman's. I mean, how would you like it if you came home and discovered that an old friend had, for example, signed your son up for Little League without his or your knowledge, under the theory that he'd make a good ballplayer, 'cause he'd seen the two of you playing catch in your backyard? It's the same basic thing here - being Green Lantern is a large order of magnitude above being Robin. You're not just back-up for another hero in a city which you're both intimately familiar with, you're responsible for the entire EARTH, plus a large sector of space. It's not something a twelve-year old should be saddled with.