Pekin Duck Laying Down a Lot, Doesn't Seem Sick

Hi all. My pekin duck Sammy has been laying down far more than usual lately. Sammy is 5 years old, I've had him all his life, and he is otherwise healthy. I noticed him laying down and straining a little to stand about a month ago. I responded by giving him warm baths for a few days and he did a lot better, not straining as much but still just laying down a lot. He has not broken any bones in his foot, there is no evidence of an infection, and he does not have bumble foot. I thought perhaps he had pulled a muscle or something. Soooo, I was wondering if this is something that happens to ducks his size? He looks like we might be part Ayelsbury (thought I'm not sure) and he weighs almost five pounds. He has always seemed to lumber around. He went to the vet a few months ago for something else and had his blood drawn, so I know there weren't any underlying issues then. I guess I'm just stumped. For those of who have older pekins, is this normal? He has had a runny eye off and on since I noticed him laying down.
He's eating normally, poop looks fine, he's drinking water, swimming normally.
Thank you in advance!

Ducks are tough to diagnose sometimes, even with a vet's help.

He could have an infection, possibly parasites… You might take a stool sample to the vet and have them check that end...

Is he part of a flock or a single duck? The baths might be good to try again, along with some poultry vitamins with electrolytes and probiotics to give him a boost.