how to reply a professor is not available for research guidance
You should not reply to this at all. He has made it abundantly clear that he is not interested in helping you in any capacity. This is actually nice of him: many professors would not reply at all, or would make promises and then not follow through. 


  I want to know if this professor can suggest my name to his colleagues.


Consider just approaching his colleagues yourself. Or, if you have the opportunity to speak to this professor in person (e.g., office hours), you could ask then if they could recommend someone to advise you. But replying to this e-mail is unlikely to have the desired effect.