he's certainly well within his rights to speculate about an aspect of Jesus' presence and life. There's little known of Jesus other than from the Bible, and this book is just his opinion, after all.
Reza Aslan defends controversial new book on Jesus
These are the arguments of academe. One or two people outside of academe might have their opinion of Jesus affected by this book. The person and teachings of Jesus have had books written outside the general Christian position many times. The big deal this time is that it has been written by a Muslim (who also believe in Jesus, but as a prophet). It will probably end up in remainder sections of bookstores quite quickly...
... of the Used, Abused, and Utterly Screwed Up.
A Secular Franciscan looks at the world... with a more jaundiced eye than ever... and lots of ellipses for you to fill in the missing text... (with thanks to Thomas S. Klise for the title)