Well he may already understand that christians consider themselves monotheistic. As a former christian, I also understand that any christian will tell you that they worship One God. Truly that is the teaching of christianity. However, a muslim cannot reconcile the worship of Jesus with the worship of God. In our eyes Jesus is not God, only a messenger of God sent to the world for guidance. Muslims do not deny the virgin birth, the revelation given to Nabi Isa (Jesus) namely the Injeel or gospel, nor do we deny the many miracles performed by Jesus. Muslims view all these things with a different perspective though. The miraculous birth of Jesus is a show of power by Allah. It only signifies for us that Allah is able to do all things no matter how improbable they are or seem to be. The miracles performed by Isa are also duly noted an accepted by muslims. From the healing of the lepers, the sight given to the blind, to the raising of the dead are all acceptable by muslims. A christian may wonder why if muslims beleive these things, do they not accept Jesus as God. Simply put, muslims know that any miracle performed by any of the prophets of Allah is from Allah. Allah has permitted that the prophet do some miracle as a proof to the people of that time that they indeed have been sent by Allah for guidance. The prophets are not acting independantly from Allah. All of the prophets including Muhammad performed some miracle during their time as an immediate proof to those able to witness it, that they were sent by God.
Furthermore, Jesus was a human being. He was born from a human mother. To my knowledge no human woman has ever birthed anything other than another human. Jesus walked around, talked, slept, ate, performed normal body functions such as urinating and so forth. From our vantage point no human no matter what status they hold can ever be elevated to God status. To us Jesus is not God in any way because of the reasons I listed. God is all powerful and is not in need of anything. Yet Jesus needed things like food to survive, shelter from the elements, water, and rest. By our definition God does not eat, nor tire, or need rest. Also, God cannot be harmed least of all by His creations. Jesus is reported to have been crucified.
The concept of the trinity is polytheism to us because it is saying that first of all God has lineage. Many christians believe that Jesus is the literal son of God. Muslims hold the view that lineage is a human and animal trait. People have families because we must procreate to survive as a species. All of the purposes of lineage that serve humans are non-issues for God. God simply has no use for such things. There is only one God, so therefore no procreation is necessary. Muslims view it as below the supreme majesty of a being such as God to have anything even slightly remeniscent of family.
The biggest reason for a muslims rejection of the concept of trinity as polytheistic is that it means one must associate a partner with God. Jesus is seen sometimes as an equal with God, or some of the unique qualities of God are attributed to Jesus. Things like the power to forgive a person is a quality that only God has. By forgive I mean the forgiveness of sins and disobediance to the commandments of God. I know from my years as a missionary baptist christian, that Jesus was seen as like a co-God with God, and we were taught that Jesus is God's equal partner. Not every denomination of christianity subscribes to this exactly, but I am using my experiance as an example.
I did not say this like I meant to say, because there is a lot of explanation missing from what I gave. I know this does not answer the question that you asked, but my goal was to give an insight as to a muslim's train of thought on what constitutes polytheism. Islam has very strict rules that guard against the worship of anything except the Creator. The concept of the trinity would never be accepted as monotheism by a muslim who understands the Islamic concept of what monotheism is. So in actuality it doesn't matter how eloquently you express this belief, if he is knowledgeable of Islam, he cannot by default accept the explanation.
Please forgive me if I have offended you as it was not my intention to do so. I simply wished to provide a more in depth reason as to why muslims think trinity is in essence polytheism, as opposed to just saying 'no we don't accept it'. It is not my intention to attack your beliefs in anyway, as you are entitled to them, nor is it my intention to tell you that you are wrong and I am right.
