There really isn't any specific established amount that I know of. And much depends on the quality of it. Any 'doggie' sources tend to be far less quality than human products, and even the human stuff varies WILDLY. Stuff that you get discount or that is simply carried mass market (grocery store, Wal-mart, etc.) isn't nearly as concentrated as the higher quality stuff.
So essentially it doesn't sound like he's getting a lot anyway.
However -- you have to understand what g/c is and does. And truly -- they still ... as in they = medical community or the holistic medical community -- haven't decided which is more beneficial. Both together or separately.
glucosamine/chondroitin are 'hydrators' -- meaning they cause the joint/connective tissue/cartilage to hold more moisture and 'puff up' -- meaning they pad better between the two boney pieces of joint. When bone rubs on bone you get inflammation. The theory is that if the connective tissue/cartilage holds more moisture it 'plumps up' and pads better ultimately lessening bone rubbing on bone and hence less pain.
THAT all depends on a lot of things -- IS there cartilage left TO 'plump up'? Or has arthritis already proceded to pretty well wear away that connective tissue/cartilage?
Is you stop giving g/c it stops helping right away. it's not very long lasting. And the amount that is in the doggie stuff is likely pretty minimal anyway (as is also probably the chewables)
You might also consider using plain Knox NutraJoint -- and I mean just the 'plain' stuff (it can also come with g/c and there's a further one that has msm in it -- but those are far pricier than the plain one). Knox NutraJoint is just calcium and gelatin (which is ground bovine cartilage). It's not a pain-killer. It simply puts the 'stuff' at the body's disposal to actually re-build cartilage.
It's NOT fast. It takes about 3 months to really make much difference. BUT the difference is permanent. Because it's not a temporary change -- it's simply helping the body rebuild what is worn out.
I'd say between 1/2 to 2/3 scoop for a large dog should be adequate given once a day. I just stir mine into my dog's food. It's not bad tasting -- tastes just a bit milky.