There is nothing wrong or strange about this man saying that he dreamt that a Muslim jinn performed tawaaf with him in Makkah and visited the grave of the Prophet (peace and blessings of Allaah be upon him). A man may see such things in his dreams, or greater things, such as seeing himself in Paradise or going up into the heaven then coming down, and other things that cannot happen in reality.
This applies if he is telling the truth about his dreams, but some charlatans tell lies and deceive people with their false dreams.
The fact that he speaks about the future indicates that he is one of the fortune-tellers or soothsayers, who deal with the jinn and find out some matters of the unseen from them, as is mentioned in the hadeeth narrated by al-Bukhaari (4701) and Muslim (2228) from Abu Hurayrah (may Allaah be pleased with him), that the Prophet (peace and blessings of Allaah be upon him) said: “When Allaah decrees a matter in heaven, the angels beat their wings in submission to His words, making a sound like a chain striking a rock. When the fear is banished from their hearts, they say, What is it that your Lord has said? They say: The truth, and He is the Most High, the Most Great. Then the one who is listening out hears that, and those who are listening out are standing one above the other” – Sufyaan [one of the narrators] demonstrated with his hand, holding it vertically with the fingers outspread. – “Perhaps the meteor will hit the listener before he can pass it on to his companion, and burn him, or perhaps it will not hit him until he passes it on to the one who is below him, until they pass it down to earth – or perhaps Sufyaan said: until it reaches the earth – and is put in the mouth of the witch and he tells one hundred lies along with it. So he is believed and they say: Did he not tell us that on such and such a day, such and such would happen, and it came to pass? That is the word that was heard from heaven. So they believe him because of one thing which was heard from heaven.”
And al-Bukhaari (7561) and Muslim (2228) narrated that ‘Aa’ishah (may Allaah be pleased with her) said: Some people asked the Prophet (peace and blessings of Allaah be upon him) about soothsayers. He said: “They are nothing.” They said: O Messenger of Allaah, sometimes they tell us something that turns out to be true. The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “That is a word from the jinn that the jinni snatches, and he cackles it into the ear of his familiar as a hen cackles, but they mix more than a hundred lies with it.”
If this man claims to have knowledge of the unseen then he is a kaafir, so beware of going to him, because going to such people and believing them is haraam, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever goes to a fortune-teller and asks him about something, his prayer will not be accepted for forty nights.” Narrated by Muslim (2230).
This applies if he goes to him and asks him something and does not believe him. But if he believes him, then the Prophet (peace and blessings of Allaah be upon him) said: “Whoever goes to a soothsayer or fortune-teller and believes what he says, has disbelieved in what was revealed to Muhammad (peace and blessings of Allaah be upon him).” Narrated by Ahmad (9532) and al-Haakim (15); classed as saheeh by al-Albaani in Saheeh al-Jaami’.
Thus it is clear that it is haraam to go to fortune-tellers and believe what they say.
Advise your brother and explain that to him, and warn him against keeping company with this man or believing him.
We ask Allaah to protect us and you from all evils and temptations.
And Allaah knows best.