I have a two year old stud colt who has had an abscess in his back leg above his hoof for just under 4 months. Recently, I've been soaking it in an Epsom salt/iodine providone mixture. It just broke open in the front, but his leg is swollen almost up to his hock. Could an abscess like that leave scar tissue or any residual problems? He is a frenchmans guy grandson and I wanted him for a barrel horse prospect. 

At thi point, I'd wonder if a foreign body was somewhere in there.

You're risking joint infection at this point. A generic abscess won't last that long, there's something there that shouldn't be. We had one get in the fetlock, lost the horse because there was too much infection in the joint. 

Cellulitis might be a concern long term as wel. 

Anytime you have an abcess with swelling above the hoof it's a good idea to put the horse on a week of Uniprim. It usually means the infection is working its way up the leg and is not limited to the hoof. Antibiotics are not a bad idea for any sub solar abcess to prevent coffin bone damage.