I am so sorry to be reading this. Dasher sounds like a loveable, goofy boy like most labs are. I am not a huge fan of pain killers for dogs unless it's a particularly painful day for them as they build a tolerance so fast and there isnt much wiggle room for increase in dosage. A medication that worked really well for my mastiff when he was going through osteosarcoma (a beast of a disease, incredibly painful and quick to metastasize) was gabapentin. Its works along side a pain killer with the nerve receptors in your brain to dull or completely wipe out any pain they may have. It also has a huge range of safe dosage, which makes it easy to increase if they need a little extra. Might be worth asking your sons vet about. There are also injectable medications like apoquel that can be helpful for older dogs. There are also places that will take blood from the patient, spin it down, and inject the separated plasma into the affected joint(s). Newer treatment, but can be extremely effective.
That being said, I still am not a fan of prescriptions meds at all, so I have lots of supplement recommendations. Hope that's okay! I use a company called Springrime Supplements. They are a small company and their supplements WORK. For Dashers case, I would recommend the joint health chewable, j-flex concentrate, advance joint health, or longevity. What's amazing about this company is that you can use all of these supplements in succession, or by themselves and you dont have to worry about overdosing. My pitbull was trambled by a horse as a puppy and it completely busted his shoulder. He couldn't put weight on it after it healed for the majority of the winter, and certainly couldn't run and play like a young dog should. I gave him these tables at 3x the dose for 1 week and there was marked improvement. Even more so after two weeks. Might be worth a try if you can "buy" some time.
Lots of folks also swear by CBD oil, which can work if you get a reputable brand. New York Bully Crew makes a high concentration one that works nicely. Most dispensaries, if cannabis is legal in your state, also carry very good CBD options. They also will carry tinctures that also have some THC in it. They have been using CBD and THC tincture combos for years in Europe on canines and it has been extremely helpful for many patients. If you elect or are able to go this route, I would recommend a ration of CBD:THC of 3:1 or 5:1. Makes them not so sleepy with less THC.
Sorry to be long winded! Animals are where my heart is, so I definitely know the pain you and your wonderful wife are in. Not only is it so difficult to watch something you care deeply for navigate life in pain with no clear light at the end of the tunnel, but trying to help and having your assistance be negated is frustrating and soul crushing. I understand wanting to pull out the stops. I adore my dog, he is my soul mate, he knows what I'm thinking, he takes care of me and keeps me smiling. Seeing him in any state other than 110% is bothersome. I understand. I sincerely hope you can get your son to consider relinquishing ownership to you so you can try a little bit more for Dasher. However, if the time comes and sweet Dasher is to meet the rainbow bridge, there are veterinarians that travel to euthanize. A bit more expensive, but more comfortable for everyone involved. It's the only way I will consider making that decision for my precious fur babies. It really makes it easier, if it can ever be easier. Also, know that sweet Dasher will never be far. I like to believe that they leave the planet for a little while and watch you...your needs, your grieving...and they come back to you. Whether it be a new friend you meet at exersice class, or a lost dog that finds it's way to you, some flowers that bloom when they shouldn't, Dasher will never be far. Call me foo foo or hippie if you want, but if you really try you will see and meet him again.
I hope the very best outcome in the scenario. I really do. You spread your kindness and I wish the same to be done to you. I hope you get more time, quality, pain-free time, with Dasher. Please let me know if theres anything else I can do or share.
Much love,
Jill