Since there are 6, put 6ml over but you will still not know if he's getting enough. You can't separate him with a buddy? feed them a treat with the b complex over it then let them out with the other 4?
Sorry It's taking me a while to see these my dh is in the hospital.
@furbabies86 is your duck still on the niacin? liquid B complex? I'd also get some vit E capsules and open one up and squeeze over his feed. Do this and lets see if it helps. along with the B complex.
I don't know how long you...
Is your drake a Jumbo Pekin? might be that is why he's having trouble with his legs. It's hard enough on the Large breeds but Jumbos bless their hearts. Hopefully getting him on the B complex will get him going again.
How'd they do last night with going inside.
The ramp isn't steep? yep takes them time to get used to things but if you are consistent they will learn.
You could let your drake be the last one in and then you can make sure he doesn't hurt himself.
Walk slowly behind them and herd them inside. If they scatter get some help. Pick each one up and put them inside and close the door. If you can't herd. They will get use to it just keep the routine the same.