Poor boy....
When I had a nasty stare fall a few years ago, I developed a couple of very large haematomas in my left shin (where the sharp edge of the concrete front steps banged my leg, a few weeks later I wound up with cellulitis of the left lower leg and when this refused to respond to oral antibiotics and the upper shin sprung a leak (puss started coming out just below my knee) I was hospitalised and my leg opened up (an abscess was found by ultrasound) and it was removed (under a local - yes it still really hurt HEAPS !!!), and I was kept in the high dependency surgical ward (one step down from intensive care) and was put on intravenous broadspectrum antibiotics), cellulitis and leg swelling still refused to respond, sent to the big regional hospital for a leg scan and another abscess found, this was removed under a general, and in both cases the surgeon left my wounds OPEN to allow them to drain, important that abscesses be allowed to drain continuously (in my case a surgical wick in the deep abscess, and the wound was packed daily for the upper one).
Is that bit of clear plastic near the stitches a drain tube or just a plastic "wick" to keep the wound more or less open to allow puss to be removed ? Leave it in.
From my reading , the abscess should be left open and just packed with sterile antibiotic treated swabbing , else it will close up was the skin heals and it will reform if the bacteria are still present in the blood or tissues.
When we thought Puff had an abscess on the back of his head (a mis-diagnosis in his case as it turned out being an aneurism on his left jugular - he never woke up after surgery
) the vet was planning on leaving the open wound open and just putting antibacterial (butadiene soaked) swabs in the hole and then a surgical island dressing over it, followed up with suitable intramuscular antibiotic theropy until the infection cleared and to let the would heal from the outer edges in (he puss will accumulate and keep on reinfecting the tissues via the blood) .
Perhaps your vet needs to take some blood do some fresh cultures) and work out which exact agent is responsible for the infection (and hard to get rid of abscess) and to help him work out the most effective antibiotic or mix of antibiotics to kill off the germs.
Or perhaps there is another abscess somewhere else (hidden) that's keeping the blood infected.
Also - are you using anything to kill off any germs on the surface around the wound (when you squeeze it to get the puss out) , 1/4 strength butadiene wash / soak (for 10 - 15mins using a soaked surgical swab) will help. as will giving him a soak in a weak solution of butadiene (enough drops in the water to make it look like weak tea in colour).
Might be worth getting the vet to show you how to use a syringe to wash the cavity where the abscess was using a syringe filled with butadiene and inserting a needle into the place the puss is coming from. (That will be scary to do IMO.)
If you visit a pharmacist , they'll likely be able to sell you a "once only use sterile woundcare kit" (includes sterile plastic tweezers (2 or 3), a bunch of cotton surgical swab (3 in x 3 in) , a blue plastic sheet and plastic dish to prepare the soaked swabs in, a bottle iodine solution and some sterile surgical wound swabs (in sealed packets). You'll also need a syringe (10ml is a good size) and a some NON-SHARP aspirating needles to suit the syringe - essential draw in 2 - 3 ml of concentrated iodine (butadiene) , the add cooled boiled water (which will be sterilised by the boiling , or sterile saline). soak the swab.
Place soaked (NOT DRIPPING WET SOAKS, BUT SOAKED AND WET) swab (or piece cut to size using sterile scissors) on the skin near over the whole the puss comes out off and leave it their for 5 - 10 (or longer) minutes while giving you sick / injured pet a cuddle and some loving.
Wash off with a swab soaked in sterile saline or cooled boiled water.
Probably do this each day to help with getting rid of the bad bacteria on his skin.
I'd be inclined to leave wound without a dressing over it unless he's a digger in which case you need to put a barrier island style dressing (Hydrofilm Plus is water proof, or Opsite or some surgical self-adhesive hypafix tape) over it to keep it clean. These come off very easily with a soak in luke warm water (may as well be very weak butadiene solution body soak - DON"T LET HIM DRINK IT !!.
Maluka honey is probably going to help applied to the surface around the wound , or maybe F10 ointment (will definitely kill of the bugs on the surface).
Hope he comes good and you can resolve his abscess problem.