Thats a typical blow out spot - some gravel may have worked it's way in and up..
Yes, it is a common spot for an abscess to blow out. But qtrhrs, you misunderstand the term 'gravel'. It doesn't mean that actual gravel has somehow got into the hoof at all. 'Gravel' is just an antiquated term for an abscess, usually one that breaks at the coronet.
I agree not to give him bute. While the inflammation is painful before it breaks, it's the body's defense mechanism, enabling the crap to be forced out. So if a painkiller is necessary, a non-anti-inflammatory one is best. I would also use a *mild* cleanser, like watered down tea tree, salt in water(epsom or otherwise, but I wouldn't soak & further soften the hoof), apple cider vinegar, rather than betadine or such. Betadine is great for killing infection, but it also kills healthy tissue, so should be used minimally & not left to soak in.
I'm glad it was just an abcsess...
'Just an abscess' is not necessarily 'just' a minor problem. You say he's resting his foot and due for a trim, so I won't comment on what apparent shape & issues(aside from the lami rings) until I see some better pics(pref. post trim). While abscesses can be one off events, such as if he stood on a particularly sharp rock on unconditioned feet, was nicked by a nail, kicked a rock with his hoof wall particularly hard, they are generally an indication of an unhealthy foot, particularly when they are 'gravels' & burst at the coronet. The reason I think it's common for them to break at the quarters is because, especially with shod horses, the quarters are left too long(in order to be level with heels & toe) so there is a lot more stress there. As evidenced in this foot by the distorted hairline/growth rings.