You don't have to put him down. You can rig up an easy support device.
Buy one PVC bar. Buy 2 L join PVC ends. Place the two ends into the ends of the main bar. You'll now have something that looks like a long stretched U.
Buy two rubber straps normally used as tie downs on car roofs.
Take out the S connectors. Make a loop with each strap and insert a circular keychain loop (heavy duty type).
Place each strap under his back legs and slide up. Now place the bar between the two looped straps.
You can now walk him easily and he'll have the harness for support. Sometimes by doing this you can rebuild the muscles and joints after about 3 months of being able to walk good again.
E-Mail if you have questions on rigging it up.
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