All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
If this man believes in the Oneness of Allaah as the Only God and Lord and believes in His Names and Attributes, then he has fulfilled the belief in the Oneness of Allaah, which is the first pillar of the two testimonies of faith in Islam. This is not enough without the second pillar of the testimony of faith, which is to testify that Prophet Muhammad is the Messenger of Allaah, sent to mankind with the divine message and legislation; that he is truthful in whatever he communicated from Allaah; that obedience to him is obligatory in all his commands and prohibitions; and that one should not worship Allaah except with what His Messenger taught to us.
If he holds these beliefs and proclaims the two testimonies of faith, then he has become a Muslim.
You should know that one of the manifestations of the beauty of Islam is the consideration of the state of necessity both in fundamentals and in subsidiary matters. When a Muslim is compelled into declaring explicit disbelief, then there is no harm in that and he bears no sin for proclaiming disbelief under duress as long as his heart is content with faith. Allaah, The Exalted, says (what means): {Whoever disbelieves in Allaah after his belief except for one who is forced (to renounce his religion) while his heart is secure in faith. But those who (willingly) open their breasts to disbelief, upon them is wrath from Allaah, and for them is a great punishment.} [Quran 16:106]
As-Saʻdi wrote, "When a Muslim is forced to commit disbelief, but his heart is reassured with faith, he bears no sin for that. It is allowable for him to proclaim disbelief in case of coercion..."
The point is that if this man cannot practice Islam openly for fear for his life and safety so as to escape murder, persecution, or considerable harm, then there is no harm in keeping his Islam a secret and performing the manifest Islamic rituals and acts of worship in secret as well. However, if the harms incurred by proclaiming his faith and practicing Islam openly are tolerable, then it is impermissible for him to refrain from practicing Islam and performing the manifest Islamic rituals and obligatory acts of worship.
In conclusion, if this man proclaims the two testimonies of faith with true and sound conviction but could not proclaim his faith and practice Islam openly for fear of significant harm, while carrying out the Islamic rituals and obligatory acts of worship secretly to the best of his ability, then he will hopefully be admitted to Paradise like other Muslims.
Allaah, The Exalted, says (what means): {And a believing man from the family of Pharaoh who concealed his faith said...} [Quran 40:28] This man was a believer, as clearly stated in the Quran, although he hid his faith from Pharaoh and his people out of fear of their harm. Moreover, the Prophet said to Abu Tharr when he first embraced Islam, "O Abu Tharr, keep your conversion to Islam a secret..." [Al-Bukhari]
Allaah knows best.