Free choice feed ALL day long... and collected at night is my choice.
If it is a formulated ration and not something being fed as a treat... the ducks should only eat to meet their ENERGY needs... which will vary according to weather, pasture conditions, parasite load, personal foraging interest, and other things.
The ONLY way ducks are going to consume less than chickens is if they are foraging more or if they are smaller. They may be getting more pasture bugs or the chickens may be lazy and just hanging by the feeder. But generally speaking ducks are larger animals than chickens and will require more feed.
How old is your drake? Is he maybe approaching molt or something?
I found if I took feed out more than once per day... the birds acted like it was treat time and pigged out. If just once per day, they just ate the feed whenever but not as excited as when I hand deliver it.
Worming *may* be in order, or a fecal float to check load. Hope he is well and you are just vigilant at noticing changes!