I think my cat might be nearing the end and is losing weight. Would it be okay to switch him to canned food to help put on weight?

My cat is 16, and his whole life he's been at around 10 lbs. He's always been range fed with dry cat food and has had the same kind his whole life. He's a Maine coon mix and so he has a lot of fur. I noticed today that his face started getting leaner, and his waist is getting extremely thin. If you make a "C" shape with your hand, the wide part by your palm is where his spine is, and where it narrows is his waist. His personality is unchanged. The vet we love doesn't have an availability until next Tuesday. In the meantime, I have been thinking about switching him to a fattier, more filling canned cat food. Does anyone have any suggestions as to which word help him put on the weight and not be hard on his old stomach? His teeth are in great condition, so if I had to feed him soft food for the next several years, it won't be a problem.