My boy was also put on gabapentin just before he turned 16. He needed it for neuropathic pain and focal seizures. The vet said he could stay on it once we got the dose right.
The review in 10 days will probably be to see how he does on the dose, as it can often need a bit of tweaking to get it right. Not sure whether your boy has started it yet, but it may help to know that they can appear off-balance and a bit druk/confused for the first few days. I panicked my boy was so badly affected, but after a few days that settled down and it definitely helped him. Also important to know that it’s quite sedating, so they do tend to rest/sleep more on it.
Sadly we lost my boy at 16.5, but he was on it for several months before that and I definitely think it gave him a better quality of life in his last few months.
At 16 I do think it’s important to have the really hard end-of-life care discussion with your vet if you can. I know it’s awful, but it does help you to come-to-terms and get a better idea of what to expect and how to manage things. In my experience, avoiding thinking/talking about it just makes it 1000 times worse when the inevitable happens and often ends with an emergency and all the shock and distress that comes with that.
Our boy had monthly monitoring appointments for his last 18 months with regular med tweaks etc, to keep him comfortable and when we did have to say goodbye, at least we had the comfort of knowing he had had a couple of years of being totally pampered and being kept comfortable and out of pain after a long, happy life. We lost him in March and I still miss him dreadfully, but there is some small comfort in knowing we did everything we could for him.