Keep in mind he's your customer. Not your boss. You always have the option of not taking the job if you think the demands are unreasonable, or if you think the compensation isn't worth the work you need to put in.

But you could start by asking him what he means by "from scratch". And, after he explains it, ask him why he has those reasons. If you want to be cynical, ask him whether you should create your own database, programming language, and perhaps even the OS from "scratch", or whether it's ok to use existing software.