Jen,

Just a thought here from another IR/PPID horse owner:

 With the swelling "down the back between the knee and hoof" maybe there's a possibility that the pain is from his recent trim lowering the heels more than normal? Even if it's just one leg/foot, maybe lowered heels stretched/stressed those tendons and ligaments from the knee  on down to the hoof?  I've found that with my older horses especially, ANY changes in hoof angle have to be made VERY gradually or it drastically affects their movement. And I've had  picture-perfect, gorgeous looking trims be absolutely not-right for my old guys. But,  maybe by now  you've confirmed that it's an abscess. 

Good luck and best wishes,

Sara, 2011, NE California