I would say あなたを見ていると彼のことを思い出します。I wrote the formal version and it's not the direct translation of "you remind me of him".
(all in hiragana: あなたをみているとかれのことをおもいだします and in Roma-ji: anata wo miteiru to kare no koto wo omoidashimasu)
Anyway, I hope this was helpful.