An abscess is fluid that is trapped underneath the sole. This fluid arrives there from several sources. Horses have a colony of bacteria that roam their body and locate themselves in trauma sites. A byproduct of their existence is gas and fluids. Cells are 90% water and once they die or rupture from injury the fluid is released and the bacteria begin the process of removing the organic material.
The easiest way to explain the process from this point is to take a piece of glass and place a drop of water on the glass. It forms a nice drop. Now take an additional piece of glass and place it on top of the first. You will see the drop spread dramatically. This hydraulic action is very powerful and can pull away the union between the sensitive laminae and insensitive sole. The experience would be much like you having a blood blister under your thumbnail and then trying to bear all your weight on that finger. It would be extremely painful.
In your horse the sole and ground is the first piece of glass. The fluid is the water released from damaged cells and from the bacteria that have localized in a trauma site (bruising, sole puncture or abnormal weight bearing. The second piece of glass is the bone column.
When your horse steps down on that foot the fluid is spread and that hydraulic action tears away at the connection between the sensitive and insensitive sole, causing the pain your horse is experiencing. The more tearing that occurs the more fluid that is produced, thereby causing more tearing that…you get the picture.
Some abscesses will discharge through the bottom of the foot, frog or buttress and others will travel up the union of the sensitive laminae and the hoof wall and discharge at the coronary band.
An abscess that discharges at the coronary band is called 'gravel'. In the late 1800's they began to put chipped rock on roadways. Many horses abscessed from the trauma of sharp rocks. Soles were left extra thick to protect the horse so the abscess material would travel up the union between the hoof wall and the sensitive laminae and discharge at the coronary band. At that time it was thought that a piece of rock, 'gravel', migrated up the wall and came out at the coronary band. It has since been learned that, in a gravel, there is in fact no foreign object that migrates up the wall, just abscess material.
You said that your "…horse is standing on soles of feet…". The horse's sole is not designed to bear weight. When a horse stands on his sole the sensitive lamanie (blood vessels, nerves, etc.) is trapped between the bone column and the ground. During weight bearing capillaries are compressed and they will rupture. Cells will die. The dead cells release fluid, bacteria accumulate, creating the scenario for an abscess.
It is imperative that your horse gets shoes and probably pads. The shoes are necessary to elevate the sole off the ground and prevent the pinching of laminae between the bone and ground. The pad will reduce the amount of pressure placed on the sole from rocks or uneven ground.
The shoe should be concaved so that the sole is not touching the shoe, just the hoof wall. If your farrier says that there is not enough hoof wall to nail a shoe to then change farriers. With today's acrylics, polyurethanes and variety of shoes and nails available a competent farrier can hang a shoe on just about any foot.