i've fed him spinach before, in small amounts in the snail jello i made, with no ill effects but i'd rather not take the chance. he's eaten cabbage before, as well as romaine and leaf lettuce, kale, and collards. all are naturally good sources of calcium, which is why he gets them so much- or at least, when he doesn't have a good supply of floaters and i don't have the time or money to find more for him lol
it's really sweet of you to offer this, and i'm sure he'll appreciate it as much as i do =)