I don't know if what he's saying is true. It seems quite suspicious to me.
I'm not sure if he's telling the truth. It seems rather dubious to me.
この場合、次のような言うのが自然かなと思います。
ーI don't know if what he's saying is true. It seems quite suspicious to me.
「彼が言っていることが本当かわからない。私にはとても怪しく思える」
suspicious「怪しい・うさんくさい」
ーI'm not sure if he's telling the truth. It seems rather dubious to me.
「彼が本当のことを言っているかわからない。私にはとても怪しく思える」
dubious「怪しい・疑わしい」
ご参考まで！
カナダ 
