R.F.
Try an old fashioned ice tray. You can put little snacks in each one and make it colorful. My son was very active, so I would keep it on the coffee table for him to graze on. I was very careful to not put anything that might potentially choke since he is on the move. Make it colorful. Couple things to try (along with the green veggies)
Grated carrot
Small chunks of cheese
Grapes
Granola clusters
Try interspersing these things with the green veggies to make it colorful and he may pick up those green veggies without even paying to much attention. The green veggies are part of the "rainbow". I also would just remind my son that the snacks were there and NEVER pushed him to eat anything.
Another rule of thumb is to just continue to put it in front of him day after day. You never now, you might catch him at an "off" moment when he tries it.
Good Luck!