He only fasts Ramadaan and does not observe any naafil fasts

What the Muslim is obliged to do is to fast Ramadaan. It is
not obligatory to fast on other occasions, such as ‘Arafah and ‘Ashoora’,
unless one has vowed to do so. If one has not made such a vow then it is not
obligatory. 

It was narrated that Talhah ibn ‘Ubayd-Allaah said: A man
from the people of Najd came to the Messenger of Allaah (peace and
blessings of Allaah be upon him) with unkempt hair. We could hear him
shouting but we could not understand what he was saying until he came close,
and he was asking about Islam. The Messenger of Allaah (peace and
blessings of Allaah be upon him) said: “Five prayers every day and night.”
He said: “Do I have to do anything else?” He said: “No, unless you do it
voluntarily.” The Messenger of Allaah (peace and blessings of Allaah be
upon him) said: “And fasting Ramadaan.” He said: “Do I have to do anything
else?” He said: “No, unless you do it voluntarily.” And the Messenger of
Allaah (peace and blessings of Allaah be upon him) told him about
zakaah. He said: “Do I have to do anything else?” He said: “No, unless you
do it voluntarily.” The man went away, saying, “By Allaah, I will do no more
than this and no less.” The Messenger of Allaah (peace and blessings of
Allaah be upon him) said: “He will succeed, if he means it.” Narrated by
al-Bukhaari, 46; Muslim, 11. 

This indicates that the
fasting that is obligatory is fasting in the month if Ramadaan. With regard
to fasting on other occasions, it is not a sin if one does not do it. 

Al-Nawawi said in Sharh Muslim: 

This hadeeth indicates that it is not obligatory to fast
‘Ashoora’ and other occasions, apart from Ramadaan. There is consensus on
this point. 

But we should not ignore fasting on other occasions such as
‘Ashoora’, ‘Arafah, six days of Shawwaal and so on, because of the great
reward that this brings and the great status it has. Moreover observing
naafil fasts makes up for any shortfall that there may be in the obligatory
fasts. 

It was narrated that Abu Hurayrah (may Allaah be pleased with
him) said: I heard the Messenger of Allaah (peace and blessings of
Allaah be upon him) say: “The first of his deeds for which a person will be
brought to account on the Day of Resurrection will be his prayer. If it is
sound he will have succeeded, and if it is lacking he will be a loser. If
anything is lacking from his obligatory prayers, the Lord will say, ‘Look
and see if My slave has any voluntary prayers, and use them to make up what
is lacking from his obligatory prayers.’ Then all his deeds will be dealt
with in like manner.” Narrated by al-Tirmidhi, no. 413; al-Nasaa’i no. 465.
Classed as saheeh by al-Albaani in Saheeh al-Tirmidhi. 

This indicates that observing naafil fasts will make up for
any lack in the obligatory fasts. 

 With regard to your question about charity, it is not clear,
but if you meant to ask about giving charity to such a man (one who only
fasts in Ramadaan), the answer is: yes, it is permissible to give him
charity, so long as he is a Muslim who prays. The fact that he does not
observe naafil fasts is not a sin. And Allaah knows best.