I have a couple of very important questions:
1) How 'fat' is he? Fat horses are much more prone to founder than a horse in 'good' condition. 'Obese' horses and horses with a thick, 'cresty ' neck are just looking for a place to founder. They are VERY much at risk of getting laminitis and founder.
He's always been an easy keeper, and it was difficult to maintain a stricter diet for him because he was kept with my hard-to-keep endurance horse. However, I sold the other horse at the end of March, so he's been on his own for couple of months and I've cut back. Up to the lameness episode, he got two flakes of alfalfa 2x/day and a quart of Triple Crown Lite with Ultramin mixed in (I wanted to give him a little - but not much - Triple crown, so the Ultramin is needed to make sure he's getting all the minerals he's supposed to). Now, he's getting straight grass (no alfalfa, period), a little less than a quart of Triple Crown Lite + Ultramin + "Remission" (an anti-found supplement). This is what he looks like - would you consider him "fat"? He's heavy and stocky, yes, but what I've felt was "fat" my BO insists is "healthy", especially for a 3-year-old...
2) Have you checked his feet for 'heat' early in the morning when they should be cold to the touch? Laminitis and founder cause heat and inflammation in the hoof. Since front feet are about 50 X more likely to founder than all four, you can easily compare the temperature of a horse's front feet to the same horse's hind feet. They should all feel cold to the touch early in the morning.
The farrier was out around 10 in the morning and did check his feet for heat exactly like you said. There did seem to be a difference between the feet then, and I've checked since but I don't feel anywhere confident in my own evaluations. If there was heat when I checked, it didn't exactly jump out at me and I could just be making up subtle differences because I'm worried.
IF he is fat and/or IF his feet feel warm or hot, he should be taken off of all grain and only fed hay that is low in sugar content. Learn more about this at www.safegrass.org
See my first comments about the immediate diet change when he came up so lame and after the farrier had been out (within 24 hours). The diet change was good either way (I haven't wanted him on alfalfa because it's super rich here), but necessary if it was laminitis.
IF he is fat AND IF his feet feel warm or hot, he should have x-rays taken to see if there is any founder (actual hoof damage) that has already occurred and then, he should be treated accordingly.
We'll see what the vet says. T-02:00!
Thank you so much for your clear and concise response Cherie and everyone else!