The answer is for all to see:
Pickthall: faultless son.
Yusuf Ali: gift of a holy son.
Shakir: a pure boy.
Dr. Ghali: a most cleansed youth."
Abu Hurairah(A Jewish Convert / NOT a Christian) states that "Satan's 2 fingers touched every child except for the Son of Mary." Would a Jew say this, unless it was from the Prophet? Think about it.
From its root, it also carries the nuance of someone who is pure in heart, righteous, one who intends to purify, keep clean. Ancient sources in lexicons record it to also mean someone 'growing, increasing, in goodness and righteousness, or purified by nature or such as shall in the future become purified'.
"Then they both proceeded until they met a boy, then he killed him. He (Moses) said: "Have you killed a soul 'zakkiyyatan' (pure / innocent / righteous / blameless) who had slain none? Truly you have done an evil / foul thing!"
It is also useful to remember that Prophet Yayha is also attested to be 'zakatan' in the Quran (19:13)
However, in all 3 cases, the strongest word, Zakkiyya is with Jesus (Compare to Yahya). Contextualize Jesus' situations with this word and it's 3 occurrences.
This leads me to believe that Jesus was indeed the only sinless man, as far as Prophet Muhammad told us. In contrast, the Quran tells us that Muhammad sinned no less than 3 EXPLICIT times! @Owari, you also beleive the 12 imams are sinless as well. But Allah only names one explicitly in his manifest book, 'Isa ibn Maryam.
Why does he have that honor?
With respect, this is the wrong question. He is repeatedly described as the Christ, Al-Masih, but most muslims do not understand the implications of this word. Look at the earliest tafsirs to understand what the Earliest Muslims really believed(Like Razi).
What was the purpose of him being sinless?
Who knows? Maybe he will intercede instead of Muhammad. After all, the Quran does not state WHO will intercede(If there will be any). He will also be a 'sign on the last day.' This leads to huge theological and philosophical debates that no one has time for. The fact is, he is sinless. Salaam.