Sorry, but that's asinine. 600 of gaba?! And that's a lot of trazodone too. I don't know what it is w/ the medical community seeming to rely on gabapentin now instead of actual pain medication. There is tramadol, and morphine if needed. Read up on gaba dependency--I think you need to be on it for a while at a high dose to actually become dependent on it. You can at least cut back to 100 mg/day until you can safely take him off of it altogether. Maybe ask the vet for tramadol instead, although at day 8 he really shouldn't be needing pain meds. I'd cut the trazodone back to 1 50 mg tab at a time. I have an old bottle of trazodone for my last male, who had some anxiety issues w/ thunderstorms (Thundershirts didn't work). The instructions from my vet were 2 (50 mg) tabs every 8 hours as needed for anxiety. So you can certainly play w/ the dose and timing, and see how he does. And don't throw those old medications away. Except for antibiotics and epinephrine, the majority of drugs do not lose much potency over time.
The incision looks fine. I imagine he'll have the stitches out within a couple days? Or are those absorbable sutures?
My first doberman girl was positive for von Willebrand's, so the vet didn't want to give her rimadyl at her spay. The vet told me, "If she is in pain, I can prescribe morphine". Well the vet closed by the time my girl was in pain, and back then they didn't have vets on-call at that office. She was trembling and crying in pain; I gave her Rescue Remedy b/c that was all I had, and it worked! Within 15 min she stopped shaking and crying, curled up and slept for about 2-3 hours. Woke up, showed more mild pain symptoms, so I gave her more of the RR. She slept all night and was pretty much back to normal the next day. So that is another option to try for pain control.