I would say that it is a combination of both. If he has been ridden hard most of his life but is now retired, those muscles lose tone and allow his spine to kindof relax down, giving the appearance of being swayed. He looks really good for his age.
It's hard to say without a side picture view, but I agree it could be a little of both.
My mom and I thought Dream had a swayback last year...
But after we moved to a barn with an indoor and could ride her alot more, we noticed her back looked better...
This is her a few weeks ago... (she's blanketed this year, which she wasn't last year in the above photo)
I'd say it looks bad again because we haven't been riding much. When we do ride, she looks really good, which reminds me I must start riding again!
ANYWAY...LOL got off track, we thought it was a swayback for a while until we noticed her back muscle up. So it could be lack of muscle and a bit of a swayback because her topline never looks quite perfect, or just lack of muscle due to lack of execise. It's hard to tell.