You did right with getting him used to the grass slowly.
Certain breeds are more prone to metabolic problems than others. The famous easy keeper.
He needs to stay off pasture until he's stable, no oats or other grain or high sugar feeds, hay only, best in a slowfeeder net. He should have a vit/min supplement or a ration balancer, and should get magnesium.
Has he been seen by a vet, x-rays taken? Do you have a capable farrier or barefoot trimmer?
If he's chubby, he needs to diet, like, yesterday.
As Shropshirerosie mentioned, Slow Feeding Horses on Paddock Paradise Tracks - Paddock Paradise Wiki
has lots of info and several interesting solutions for slowfeeders. Katy Watts | Safergrass.org
tells you everything there is to know about grass.
We have a couple of members here who can help you and guide you in the right direction with hoofcare.