You will probably have to resort to being not very nice.....he sounds like he could be a stalker in the making. Not saying he is, but there may be a possibility. Have a contingency plan and don't let him know any plans. Utilise your friends to help you.
With that disaster theory in mind. Be concise. Give him the reasons why, the real reasons. You won't do him or yourself any favours by being wishy washy and nice. They can be misconstrued as she loves me forever but is scared of our love.
Not trying to scare you....just going through to extreme possibilities....I have been stalked before by this kind of person...wasn't fun.