You've got two things working against you - the act of somehow restricting his ability to "chow down" requires physically making it harder for him to grab large mouthfuls. The only way to do this is with a hay bag or net and adjust the available space through which the hay can be pulled. This totally goes against your need to feed from the ground rather than from a hanging hay net.

I personally agree with you, the hay net is not the safest thing - I have had better luck with bags that completely enclose the hay leaving only a nose-sized hole through which they pull the hay.

the very only thing I can think of may be a grazing muzzle. This is a muzzle made of nylon and plastic that goes over the nose and has a strap over the ears, like a halter. There's a small hole in the middle of the plastic cup - the intent is that the horse pushes it on the grass and gets to only bite what pokes up through the hole. Not sure if this will work with hay though. You might be able to make the hole larger so the hay can poke through but he can't grab a mouthful at a time.

Good luck!