At this age, I do not doubt there is some degree of DJD as you suggested. Whether that is in the hip or knee or elsewhere doesn't matter as much right now.
But, keep in mind, even if he gets back to normal, there still could be a underlying problem like a cruciate ligament taer in the knee. So, veterinary examination is important.
Yes, you can start a supplement. Don't expect any improvement for nearly 2 months!!!!
Yes. It can take well over 4-6 weeks or longer before any type of chondroitin, glucosamine, MSM, etc...is of any benefit.
How to dose:
My suggestion: Purchase Dasuquin. It is over-the-counter and carried by quite a few vets. Excellent product with dosing directions on the label.
But, you need more than that. You'll want the vet visit so you can get Boomer on to a non-steroidal anti-inflammatory like Rimadyl, Dermaxx, Previcox or Metacam to help out. This is by prescription only and may work wonders for him!