If a person sneezes several times in a row, when is it prescribed to say “Yarhamuk Allah” to him?

My brother sneezes 5 times in a row! do i have to say yarhamukullah
after every sneeze, or can i just wait and say it once at the end?.
Praise be to Allaah.
Saying "Yarhamuk Allah (may Allah have mercy on you)" to a person who
sneezes, if he says "Al hamdu Lillah (praise be to Allah)," is a
communal obligation, in the sense that if one person says it, the
obligation is waived for the others. If no one among those who hear
him says it, then all have sinned, because of the report narrated by
Abu Hurayrah (may Allah be pleased with him) who said: The Messenger
of Allah (blessings and peace of Allah be upon him)said: "There are
five things that the Muslim must do for his brother: returning the
greeting of salaam, saying 'Yarhamuk Allah' to one who sneezes…"
Narrated by Muslim (4022)
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: Our
companions and others differed concerning visiting one who is sick,
saying "Yarhamuk Allah" to one who sneezes and initiating the greeting
of salaam. What the texts indicate is that these things are waajib
(obligatory), and it is said that it is a communal obligation.
End quote from al-Fataawa al-Kubra, 1/443
Secondly:
If a person sneezes more than three times, you should not say
"Yarhamuk Allah" to him; rather you should pray for well-being for
him, because of what is narrated in al-Muwatta' (1521) from 'Abdullah
ibn Abi Bakr from his father, that the Messenger of Allah (blessings
and peace of Allah be upon him)said: "If he sneezes, say 'Yarhamuk
Allah' to him; then if he sneezes, say 'Yarhamuk Allah'to him; then if
he sneezes, say 'Yarhamuk Allah' to him; then if he sneezes, say: 'You
have a cold." 'Abdullah said: I do not know whether that is after the
third or fourth time.
And it was narrated that Salamah ibn al-Akwa' (may Allah be pleased
with him) said: The Messenger of Allah (blessings and peace of Allah
be upon him)said: "The one who sneezes may be told Yarhamuk-Allaah
three times; if he sneezes more than that, he has a cold." Narrated by
Ibn Maajah (3704); classed as saheeh by al-Albaani in Saheeh Sunan Ibn
Maajah.
An-Nawawi (may Allah have mercy on him) said: If a person sneezes
several times in a row, the Sunnah is to say "Yarhamuk Allah" each
time until he reachesthree. We narrated in Saheeh Muslim, Sunan Abi
Dawood and at-Tirmidhi from Salamah ibn al-Akwa' (may Allah be pleased
with him) that he heard the Prophet (blessings and peace of Allah be
upon him) say, when a man sneezed in his presence, "Yarhamuk Allah."
Then he sneezed again, and the Messenger of Allah (blessings and peace
of Allah be upon him)said: "The man has a cold." This wording is the
report narrated by Muslim.
As for the report narrated by Abu Dawood and at-Tirmidhi, they said: A
man sneezed in the presence of the Messenger of Allah (blessings and
peace of Allah be upon him) whilst I [the narrator] was watching, and
theMessenger of Allah (blessings and peace of Allah be upon him)said:
"Yarhamuk Allah." Then theman sneezed a second or third time and the
Messenger of Allah (blessings and peace of Allah be upon him) said:
"Yarhamuk Allah, this is a man who has a cold."
At-Tirmidhi said: A saheeh hasanhadeeth. End quote from Kitaab al-Adhkaar, 1/273
Shaykh Ibn 'Uthaymeen (may Allah have mercy on him) was asked: Is it
obligatory to say "Yarhamuk Allah" to one who sneezes once, and if he
sneezes three times to repeat that three times or more?
He replied: If he sneezes three times and you say "Yarhamuk Allah"
each time, then after the third sneeze you should say to him: "
'Aafaaka Allah (may Allah grant you well-being); because that is a
cold, so say: May Allah grant you well-being; you have a cold. All you
have to say is: May Allah grant you well-being, and you may add: You
have a cold.
End quote from Liqa' al-Baab al-Maftooh, no. 127
And Allah knows best.