Thanks for the information. It's very helpful.
This honestly and truly sounds like Lyme or Anaplasma to me. The two big symptoms your dog has: transient lameness in the limbs and the sudden onset lethargy are both huge red flags when it comes to diagnose these tick borne diseases. There's a website that outlines some of the symptoms here: http://www.provet.co.uk/health/diseases/lyme.htm
One of this problems is that a dog can have a silent infection for years before they become symptomatic...which means he could have gotten a tick on him last summer, it could have transfered the disease and it could have simply been dormant all this time. Since tick borne disease can lay dormant, it's often triggered with another illness or something like surgery that can impact the immune system, which gets the illness to flare up.
I did some research while I was waiting for your reply and did find that tick borne diseases are on the rise in Edmunton, Alberta. You can read more about it here:
I think the best thing to do is to have him tested for tick borne disease. In my opinion, one of the best (and fastest) options for that is the 4DX test. It only takes 3 drops of blood and 8 minutes for you to have a 'for sure' answer. You can read more about 4DX testing here: http://www.idexx.com/view/xhtml/en_us/smallanimal/inhouse/snap/4dx.jsf?SSOTOKEN=0
I strongly suspect he's going to come up positive. Then it's just a few weeks (usually 31 days, actually) of Doxycycline, and he should be back to normal.
I hope this helps.