Yes, I did it to my pony a couple of years ago - don't feel bad,it happens. But do learn from your experience
I agree that he should come off the oats pronto.
As to the grass & the hay;
What he ideally needs is lots of low protein fibre. I don't know what your hay is, so I can't comment on whether it is helping him, or making him worse. If it's rich hay, then get him off that too. If it has alfalfa it it, get him off it yesterday!
These are the basic rules for a laminitic:
- once a laminitic, always a laminitic.
- learn to check the hooves daily for heat, and learn to check for a digital pulses which is an indicator of something amiss in the foot.
- slim is good, slim and fit is best
- nighttime grass is better than daytime grass
- rough grass is better than lush green grass.
- involve your farrier, and vet if necessary
Have a look at Paddock Paradise on the web. It's a horse management system that advocates putting in a 'track' in your pasture that will restrict horses grass intake, but keep them moving in a natural way. I'm going to be doing this in my pasture this summer with temporary electric fencing. Essentially, the horses will be on a track around the perimeter of my pasture, and the inner circle will be growing ready for winter feeding.