Recurring abcess in the shoulder

My mom bought my little sister a 14 year old morgan this past summer for a steal of a deal. The lady said that he was too upity for her trail riding business but we have since then began to wonder. He was great for the first several weeks (calm as an old nag for my mom but with just the right amount of get up when my sister wanted it) but then he started to slow down. Not limp really but he just didnt really seem himself. His left shoulder began to swell and then he really did begin to limp. His shoulder became the size of large melon and we tried all sorts of massages and compresses and magic healing pens (long story there lol) before we finally gave in and let the vet operate on him. The vet drained the abcess and left in a shunt to prevent it from coming back. After he felt it was safe the shunt was removed and although it seemed to take a long time, the wound healed up well with no scar.
It has now been a few weeks since the scar healed and the swelling has returned! The vet recomended a hot sea salt compress twice a day to bring the abcess to a head but that didnt work last time so although we are giving it another go, we are uncertain it will succeed. To put it bluntly we cannot afford another surgery and my sister is very upset by the prospect of putting her horse down . I am hoping that someone out there has a suggestion that we havent tried that might help to resolve this problem. He has not fallen and his saddle is the perfect fit by all accounts (plus since the last time he was swollen we have only been riding bare-back).

Did the vet recommend any antibiotics? Maybe another round of those would bring the swelling down along with hot compresses.....Could there be something foreign in the shoulder that's causing it to abscess?

I see your in Kamloops, maybe a phone call to Henry at the Cloverdale Pharmasave might help. He's amazing and we've had excellent results with his homeopathic remedies in conjunction with regular medications.

I had a friend that had this issue and even had the vet out, it turned out that the horse had a bullet in it's shoulder that had healed and was abscessing over and over again. I'm wondering if it's some type of a splinter?

If the swelling returned than something is in there that doesn't belong. Was there just a drain put in or did he get opened up and the site explored or ultrasounded? I'd also look at a bone chip (from a kick).