My 5mo is down with a flu. He's only drinking 35% of his usual milk intake. Rejects milk for later half of the day. Shld I be worried ?

My 6mo is also down with flu since last week. Usually he drinks about 210ml per feed.. Nowadays , due to flu, he drinks only about 150ml. The blocked and runny nose and phlegm in throat makes drinking from bottle difficult . So I try to clear the mucous in his nose to make feeding a little easier.