Is it possible to speak to him on the phone later when things are quieter? It is often easier and better to discuss these kinds of things over the phone if you can. If not, then email is ok too

I would sit down and work out exactly which questions you have and how much you would be willing to accept in pay for the position. Then start negotiating. I would say that I was extremely interested in the job (and list the positives) but you wanted more information and then also ask if the pay rate is negotiable based on experience and past performance, outlining why you believe that you should have a higher rate of pay - different companies have different pay rates, even within the same field so you would need to be able to demonstrate why they should pay you more based on your skills instead of their 2nd choice who might just accept the position at the lower pay rate. This can include things such as extra study, higher work experience, working with highly regarded companies/professionals etc.

The best thing is to know your bottom line of what you will accept, and if you will still take the job if they can't increase the pay, but also understand that they may turn around and rescind the offer if they think you are asking too much of them or they can't afford the pay increase for the position. It is rare but it does happen.