My 13 yr old cat seems to be having difficulty swallowing and is now retching.?

2 days ago he seemed to be trying to swallow air all the time. Then he retched a few times. Last night he brought up a small amount of clear saliva/mucus. But he is still eating his food OK - but is taking much longer to do so. (He used to hoover it up in record time). He is sleeping most of the time.