This is a common condition in old age he's got arthritis and this causes very severe inflammation and pain. Assuming the dog is about 20 kg take these steps but you should consult your vet in person:
TO BRING A DOG BACK UP ON HIS FEET/ SHORT TERM MEASURE
- Depa Medrol : this is a 2 ml vial give him 1 x 2 days IM. This a hormone and will immediately reduce internal joint swelling and resulting pain. (available at a human chemist)
- Prednisolone injection: Also reduces inflammation. 1ML x 5 days. Lowers immunity over long periods so continous use us not recommended. Also can use 10mg tablets (avl in human chemist)
Essentially you are giving (1) & (2) to allow him to start moving again and
LONG TERM MAINTENENCE
- Joint supplement such as JointAce (human) or PetJoint (vet) that contains chondroitin (2 x morning - evening) for 3-4 days, then 1 x morning evening for life. This is to keep mobility for life.
- Highly recommended that he swim to maintain muscle tone and - and not walk long distances.
- You could use a hot water bottle or an infra-red lamp (available at human chemists) to give him some heat for a few minutes a day 2 or 3 times and massage his legs.