If he goes down to eat grass when you are leading him shake the leadrope until he puts his head back up 😁👍🏻
Thank you for all the help!
He needs to learn to focus on you, not his food 😉 I agree with Martha, persistence is really important. Set boundaries and stick to them. For example my horses are not allowed to eat if they're haltered/bridled. I did also teach them a verbal cue for "you can eat now" and "stop eating" as I need them to eat when tied up/yarded at shows.
You could use a grass rein (pretty much just a piece of rope or twine from the saddle to the top of the halter/bridle so they can't put their head down). When you are handling him you need to be very persistent and pull him back up straight away. Bit of a hard habit for them to get out of once they are in it!
I would kind of pull his head up but not be rude about it. And if you keep doing it could become a habit. Ps. My horse does the same thing: natural instinct to eat grass