Poor doggie... a torn ACL is no fun. My English Mastiff (Condi) tore BOTH of hers when she was a puppy, and I had to have 2 separate surgeries to have them repaired. 2 TPLO's (tibial plateau leveling osteotomy), as you can imagine, do not come cheap, but it so improved her quality of life, I think it was $7500 well spent.
Prior to her TPLO's, our local vet had her on a combination of Zubrin and Tramadol. While on the medications, all of her symptoms seemed to disappear. The vet warned us that they are powerful medications that masks the pain, and not to confuse it with actually being healed. It's simply a doggie 'pain-killers' of a sort.
You may want to ask your vet if your guy is a candidate for the medications, and if not, maybe there's an alternate to it that's in the same class of medication. Be sure and have your vet do bloodwork on a regular basis, because Zubrin tends to effect the liver, but it's worth a shot.
Good luck.