I can't keep up with his thinking because it's so off-the-wall.
I have a hard time following his thoughts because he's so eccentric.
この場合、次のような言い方ができますよ。
ーI can't keep up with his thinking because it's so off-the-wall.
「彼の思考はとても突飛でついていけない」
can't keep up with ... で「…についていけない」
off-the-wall で「突飛な」
ーI have a hard time following his thoughts because he's so eccentric.
「彼はすごく突飛なので、彼の思考について行くのが大変だ」
to have a hard time following ... で「…についていくのが大変である」
eccentric「突飛な」
ご参考まで！
カナダ 
