He is travelling from Riyadh to his family in the south, then he will enter ihram from their miqaat

Firstly: 

It is permissible for one who is in Riyadh and has decided to
go for Hajj, to visit his family in the south, then when he wants to go for
Hajj he may enter ihram from their miqaat, because of the report narrated by
al-Bukhaari (1524) and Muslim (1181) from Ibn ‘Abbaas (may Allah be pleased
with him) who said: The Messenger of Allaah (peace and blessings of Allaah
be upon him) defined the miqaat of the people of Madeenah as Dhu’l-Hulayfah;
that of the people of Shaam (Syria) as al-Juhfah; that of the people of Najd
as Qarn al-Manaazil; and that of the people of Yemen as Yalamlam. And he
said: “And these miqaats are for the people at those very places, and
besides them for those who come through those places with the intention of
performing Hajj and ‘Umrah. 

The words “And these miqaats are for the people at those very
places, and besides them for those who come through those places with the
intention of performing Hajj and ‘Umrah” include the scenario asked about
here. 

Secondly: 

With regard to the intention for Hajj, it may refer to two
things: 

1.

The decision to perform Hajj; there is no specific time for
this. A Muslim may decide to do Hajj from the beginning of the year, or he
may decide to do it a few days before Hajj begins. 

Many Muslims intend to do Hajj in the following year, i.e.,
one year before Hajj, and there is nothing wrong with this. Rather it is
something that is approved of because it is resolving to do something good. 

2.

What may be meant by intention is starting to do the rituals.
This is only prescribed in the months of Hajj and not before. It comes after
passing the miqaat and wanting to embark upon the rituals of Hajj by
entering ihram and reciting the Talbiyah. 

The month of Hajj are Shawwaal, Dhu’l-Qa‘dah and the first
ten days of Dhu’l-Hijjah. 

Entering ihram for Hajj before these months is makrooh and
there is a difference of scholarly opinion as to whether it is valid. 

Ibn Qudaamah (may Allah have mercy on him) said: If a person
wants to do Hajj and the months of Hajj have begun, when he reaches the
miqaat the best thing for him to do is to do ghusl. The words “and the
months of Hajj have begun” indicate that it is not appropriate to enter
ihram for Hajj before the months of Hajj have begun. This is what is
preferable, because entering ihram for Hajj before the months of Hajj have
begun is makrooh, because it is entering ihram for it before its time has
come. It is akin to entering ihram for it before reaching the miqaat;
moreover there is a difference of scholarly opinion concerning its validity.
If a person enters ihram for it before its months have begun it is valid,
and if he remains in his ihram until the time for Hajj begins, it is
permissible. This was stated by Ahmad; it is also the view of al-Nakha‘i,
Maalik, al-Thawri, Abu Haneefah and Ishaaq. 

‘Ata’, Tawoos, Mujaahid and al-Shaafa‘i said: He should make
it ‘Umrah, because Allah, may He be exalted, says: “The
Hajj (pilgrimage) is (in) the well-known (lunar year) months”
[al-Baqarah 2:197], i.e., the time for Hajj is limited to these months,
or the months of Hajj are known months. As it is proven that this is the
time for it, it is not permissible to enter ihram for it too soon, as is
also the case with the times of prayer.

End quote from al-Mughni, 3/119. 

And Allah knows best.