Oh, he's been locked up a lot in the last month, much more than previously - that could explain a lot, as AppyT pointed out. Horses really need free movement, or at least a LOT of in hand walking, if they are locked up. Lack of exercise effects digestion too, so it could have been the cause of a mild colic too.
Laminitis is the intial inflammation & degeneration of the laminae at the hairline. Founder is the mechanical progression of this, or of other problems, which causes the internal structures to 'sink' or rotate in the hoof capsule. While there may be no outward signs at all of lami - altho the horse is generally sore - it IS generally pretty clear for an experienced eye to see that a horse has foundered, if it is progressed very far at all. Of course x-rays are more accurate & precise tho.
Saying your horse is OK except on gravel is unfortunately quite normal & could signify that he has thin soles, but if he's been OK until recently when he was locked up, I'd guess heel pain is the more likely cause of his sensitivity. He may also have thrush, which can be painful. I would treat any infection, ensure he's well & frequently trimmed & give him as much exercise as possible, but boot or pad him for surfaces he's sensitive on for now.
Re the pic, can't really tell anything at all from it. The hooves are half buried in bedding for a start. You need to take hoof pics on level-ish ground, from near ground level, front- & side-on, and a couple of different angles of his soles would be helpful too, inc. sighting down from the heels, to guage depth & heel height.