finchandlovebird took the words out of my mouth. He does look rather scruffy and his feathers aren't "tight" as is ideal.
So I would also guess that perhaps he's just starting a moult and feels a bit off?
I'd offer him lots of good nutrition and a heat lamp and see if that helps. I'll be curious to see he takes to the heat lamp.
It looks like it could be the result of weak muscles. There are lots of things that can cause weak muscles, ranging from injury and birth defects and beyond.
One cause is a deficiency in vitamin D3. Was this an indoor bird? If so, that's very likely an issue, so I'd see about supplementing him. A D3 deficiency can be so profound that it literally causes birds to fall off the perch they're so weak. Sagging wings are another telltale sign.
I'd also see about getting him into a flight cage and/or giving him some time outside of the cage to strengthen those wing muscles.
I'd also offer plenty of protein to help build up muscles, assuming that's an issue.
It's also possible he's just not a great specimen. If he seems okay otherwise and doesn't seem to improve at all with good nutrition, heat lamps, etc. then that may just be the case.