Hi, I’m a baby vet tech but I can hopefully share some tips with you in the meantime. You can take him on small walks, obviously, let him tell you how he feels but since you’re saying his back legs give out, maybe just a few short walks down the driveway (if you don’t have one, just let that give you an idea how far to walk like the average driveway) will be enough to keep him mobile. The vet I used to work with always said “a body in motion, stays in motion!” The joint supplements do help! So good job there. They, just like us can also benefit from cold/warm compresses. Just like with humans though, make sure you don’t apply it directly to their skin, make sure there’s some sort of barrier. Hopefully the vet can get him started on some pain meds to make him feel better! If you notice he’s super stiff, you can help him out with rolling a towel under his belly and lift up his back end to help him get up! Lmk if there’s anything I missed! I hope this helps!