Add some pineapple chunks to his meals. That will make his poop taste nasty and hopefully make him think twice about making a snack out of it.

Not sure what to offer about the grass-eating, though. Maybe try bulking up his regular meals? My trainer suggested to one lady who was trying to get her dog to lose weight to add some thawed green beans to his meals. This adds bulk and helps them to feel full, without adding a bunch of unhealthy calories. Nel (my trainer) does this for her dogs (very svelte Dobies). She buys bags of simple frozen, cut green beans, and adds a handful to their bowls once a day.

It sounds like you shouldn't have any trouble convincing him both pineapple and green beans are good to eat.