Taking a rabbit outside to a field and not putting him on a lead would be saying goodbye to your rabbit. Even the most friendly of rabbits you may struggle to get back, as they may decide they want to run around, and they can run much faster than us! Rabbits are prey animals and designed for escaping, bolting and eluding capture. You also risk dogs, cats and birds of prey...
Rabbits are not dogs and don't do as they are told to please us, they will do as they are told only if they want to or if there is something in it for them (a reward). As such, rabbits also aren't good at being 'walked' like a dog, they don't really grasp the concept of walking beside you, the rabbit ends up walking you. Rabbits are also prey animals and territorial, so taking a rabbit outside of it's normal territory where it doesn't know the escape-routes and hiding places can be stressful for them and can make them flighty.
If you did use a harness, you should be aware that if the rabbit bolts it may break it's neck or back - rabbits' bones are very brittle as they are designed to be light-weight. There was a rabbit on here recently that choked himself with his harness when he freaked out. The jacket-style harnesses are safer than the H type, if you do use one, and they can be made safer by not using an extendable or very long lead. I wouldn't recommend using a harness in anywhere that wasn't safe and enclosed - rabbits can escape harnesses so you need to know they can't escape the garden too.
If you would like him to get some outside exercise and eat some grass, do it in a secure garden, preferably in a run. Be careful to build up the amount of time he is in there slowly, as too much grass when they are not used to it can cause bloat or diarrhea. House-rabbits can find outside scary at first, so make sure he has a hiding place in the run.