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 ?

I am so sorry that your little one is sick. Prehaps his throat hurts. Just a thought but have you tried offering cold milk as that could be soothing? When my little one had tonsilitis, it was just like you are saying - took us syringing tiny amounts of fluid in for ages.