VAHEGURU JI KA KHALSA, VAHEGURU JI KI FATEH
-How is Sikhi better than other religions?
Let's avoid the superiority debate (because everyone says their religion is better). Rather, look at the objectives of each religion. They are different from each other. Sikhi wants union with the Almighty, dead or alive. Others have aspirations of heaven, nirvana, and many other things. For a Sikh, the state of "So Satgur Pyara Mere Naal Hai, Jithe Kithae Mainu Laye Chaadaiyaee" - that you are with the Guru/Almighty at all times is paramount. Hell would be heaven for a Sikh because the Almighty is with him/her. Conversely, "Tudh Bin Rog Rajiyaa Da Odan" - the comforts of luxury and heaven are a burden and hellish if the Almighty is not with the Sikh. If this attracts the person, he should look at Sikhi. If he is really serious about this question, tell him to read the sacred texts of other religions and compare with Gurbani. [
sacred-texts.com] - religions sacred texts are located here so he doesn't have to go searching.
-He believes in God, but is confused as to why he should think of himself as a Sikh. He wants to know what does Sikhi offer for a person like him.
That is personal. Reading Gurbani will answer that question. At a point, the person has to make a choice as to whether to go towards a religious path. It is very hard to take an unwilling person towards Sikhi unless he/she has some motivation to do so. Doing Sangat can have that effect - discussions may not. Gurprasad is vital, without which the spark towards Sikhi is not ignited. Is the person someone remotely interested in Sikhi or are you trying to convince him/her to turn towards Sikhi?
-Why does God want Sikhs to Jap Naam? and how will doing Jaap of Naam benefit a person?
Reading Gurbani is a testament to why Naam Simran is vital. It is the breath of a Sikh- without which a Sikh dies. "Nanak Jit Vela Visrai Mera Swami, Tith Vele Mar Jayaoo Jeea Mera". Gurbani explains it, a Sikh lives it, a non-Sikh may not understand it. One who does Naam Simran knows the "Goongai Ki Mithiai" taste of it and cannot describe it in words.
-He thinks Sikhs have only been keeping kesh since the time of Guru Gobind Singh Jee, the Sikhs before Dasam Guru did not keep Kesh. How would one answer that?
Where is his evidence that the Sikhs prior to Dasam Patshah did not keep Kesh (there are references to Bhai Mardana Ji being told to keep kesh as one of the three requirements of following Guru Sahib. I don't know the reference, sorry). And despite that, even if the Tenth Master made it mandatory, what is the issue with that? All Sikh Gurus are one light in different bodies and now that light is in Sahib Sri Guru Granth Sahib Ji. Never mind that Gurbani speaks of "Gurmukh Roam Roam Har Dhiavaiye" - linked to Simran (see above) as to the spiritual necessity of Kesh.
If the person shows interest in Sikhi, give him Sangat and encourage him to do as much Sangat as possible. If he likes Sangat of Gursikhs, questions of Rehat and requirements dissolve and rather the individual develops a desire to follow the Rehats and doesn't care for rationalizations and explanations, as well as an inner desire for reading, understanding and living Gurbani.