I would probably just stick to "gotten rusty" in most contexts:
His English has gotten rusty. (American)
His English has got rusty. (British)
The (slightly more formal) alternatives that sound the most natural to me would be:
His English has deteriorated from years of disuse.
His English has worsened from years of disuse. (perhaps)
Unlike "gotten rusty", both pretty much require "from years of disuse". An extended period of disuse is already strongly implied when you use "gotten rusty". This doesn't quite apply to the others, even though that's generally the way one's language skills worsen.
The above are similar to your example of "He has experienced a decline in his English skills after years of disuse", but just rephrased to be less verbose. Native speakers rarely, if ever, speak like that. Also, "declined" doesn't sound quite right to me here, although I wouldn't say it's wrong either.
"Deteriorated" makes it fairly clear it's now pretty bad (probably the closest to "gotten rusty"), while "worsened" only means it's some amount worse than before, which doesn't really tell you that much. "Significantly worsened" would closer to "deteriorated", although the latter sounds much more natural.
I opted for "from" instead of "after", as that more explicitly says years of disuse is what caused it to worsen. Although most people probably wouldn't look twice if you use "after".
In an essay or academic writing, the "has experienced a decline" construct can be more appropriate. Although I would probably opt for "ability" rather than "skills", since "skills" is plural, which doesn't typically go with "decline". Also, "English skills" seems a bit informal.
He has experienced a decline in his English-speaking ability due to years of disuse.
He has experienced a decline in his ability to speak English due to years of disuse.
"From" or "after" could probably also work here, but "due to" sounds a bit better with the more formal writing and when it's not right next to "decline".