Baby not eating enough

My boy is 6 weeks today. He has had a cold for the past few days and starting last night he hasn't been eating like he usually does. He is formula fed and he usually eats anywhere from 4-6 oz per feeding every 3-4 hours. Since last night I've been struggling to even get 2 oz in him. He had one decent feeding this morning of 3 oz but apart from that he's had a couple 2 Oz feedings and 1 oz feedings. It's not like him. I'm worried he is going to get dehydrated.

He does seem a lot sleepier than usual and falls asleep almost Everytime I do try to get it in him and I can't get him to wake up and continue feeding. What should I do?