Sayyedina Khidr (Alay hissalaam) also knew the Ghayb?
Imam Tabari, Imam Qurtabi and Qadi Shawkani quote the following narration: 

Ibn Abbas reported that Khidr was a man who did all his work with the knowledge of the Ghayb
because he knew the Ghayb. 

[Tafsir Tabari, Tafsir Qurtabi, Tafsir Fathul Qadir, by Imam Tabari, Qurtabi and Qadi Shawkani].

The above statement proves from a narration from a Sahabi, which is accepted by leading and reputable classical authorities, that it is permissible to say about Khidhr that he knew the Ghayb, so how can it possibly be wrong to say that Prophet Muhammad (May Allah bless him and grant him peace) also knows the Ghayb.