He only fasts Ramadan and does not observe any naafil fasts

What the Muslim is obliged to do is to fast Ramadan. It is not obligatory to fast on other occasions, such as ‘Arafah and 'Ashura, 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-Allah said: A man from the people of Najd came to the Messenger of Allah (peace and blessings of Allah 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 Allah (peace and blessings of Allah 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 Allah (peace and blessings of Allah be upon him) said: “And fasting Ramadan.” He said: “Do I have to do anything else?” He said: “No, unless you do it voluntarily.” And the Messenger of Allah (peace and blessings of Allah 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 Allah, I will do no more than this and no less.” The Messenger of Allah (peace and blessings of Allah 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 Ramadan. 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 'Ashura and other occasions, apart from Ramadan. There is consensus on this point. 

But we should not ignore fasting on other occasions such as 'Ashura, ‘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 Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings of Allah 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 Ramadan), 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 Allah knows best.