My 88 Yr Old Father Has Begun Producing Copious Amounts

my 88 yr old father has begun producing copious amounts of clear mucus. always occurs after the evening meal. no fever or other problems. will burp at least twice then gag up very thick mucus in total amounts of 6oz or more. may last for 30-90 minutes. afterwards he feels fine; no sore throat or anything else. has done this twice this week. prior incident was appx 6mts ago. help!