Will he be up to make a whole new trilogy score?

Probably not. Kevin Kiner would be a nice replacement though, given his work on the Clone Wars cartoon series.

If Cameron isn't directing, there's no way Williams will score, though I'm sure Disney will ask him to.

He'll probably get credited for the original theme which I'm sure they'll use throughout the series but I reckon they'll get a new composer. They did the same with Harry Potter.

Unless he has the same genes as Elliott Carter (an American Classical composer who died in 2012 at the age of nearly 104 - and was an active composer right up to the end), then I'd say it's unlikely. I think Williams realizes that the odds are against it, and I think he's humble enough to pass the baton over to someone else who is younger and more likely to complete the task.

I think the only person who can tell you that answer is his doctor.

And to be fair, he'll only be 87 at the end of the trilogy, as Disney is apparently releasing the new films every 2 years.