Back Leg weakness

I have a healthy 18 year old Terrier/Daschund mix. He weights about 14lbs, down from 16 lbs the last few years. He does have arthritis in his back legs, tore both his CCL’s over 6 years ago and we did not do surgery. You can see the narrowing in his hind end from muscle loss, my vet told me that was normal and expected in a dog his age.

He is on Galliprant and Gabapentin. He’s taking 100mg of Gabapentin at night(6 years) and the last month or so he’s very unsteady with his back legs(one more than the other) for a few hours in the morning. I know Gabapentin can cause hind end weakness, and while the weakness has been there for years it’s never been this bad. He’s collapsing and having a hard time getting up. It is usually a little bit better by the evening, but the last few days it hasn’t gotten much better. We do monitor him closely so I know he didn’t injure himself jumping(he doesn’t even try to jump anymore). He is able to walk but his gait is weird because of the weakness in one leg. He doesn’t seem to be in pain, no panting or biting at his body anywhere.
All of his bloodwork is good, just had that done.

He eats great, pees outside, he does poop wherever now. I don’t think he even knows he’s pooping, that’s started about a year ago.

After all that my question is if nothing else is physically wrong with him like a broken leg or whatever, what other medication can be swapped for the Gabapentin?
He sleeps great at night, partly because of his age and more so because of the Gabapentin I think. Maybe it’s just too strong for him now. But I don’t want him to go all night without some pain relief.

I will be asking my vet, but he has mentioned Librella and I don’t want to do that. So I’d like to go in with some ideas for other meds to complement the Galliprant.