①My grandfather is getting weak in the legs.
so he doesn’t seem to go out lately... と英語の学習本を参考にして自分でも作ってみましたが
②My grandfather’s legs is getting weak.
でもいいのかな...それなら ①と②の違いは？
と思いました
My grandfather is getting weak in the legs recently so he doesn't seem able to go out.
My grandfather's legs are getting weak lately.
ご質問に書かれている
①My grandfather is getting weak in the legs. so he doesn’t seem to go out lately
でも問題ないですが、「最近出かけてないみたい」という場合は
he doesn't seem able to go out.を使うとよいでしょう。
⇒◎My grandfather is getting weak in the legs recently so he doesn't seem able to go out.
祖父は最近足が弱くなっているので最近出かけてないみたいだよ。
ご質問いただいている②のMy grandfather’s legs is getting weak.ですが
足は日本あるので、動詞はareを使いましょう。
⇒◎My grandfather's legs are getting weak lately.
最近祖父は足が弱くなっている。
＊Lately とRecently = 最近
オーストラリア 
イギリス 
