Yes, setting up an exclusive deal with him would mean that you only offered his products. That doesn't mean that is all you can sell on your site, it just means that you will not offer competing products against those you list of his.
For example, mobile phones, you would only list phones from him, however if you sold baby seats and didn't source them from him, then you would be free to list from as many suppliers as you wanted.
You can also choose to set up a separate account for his products only if you feel it is worth it, and then build your branding around his genre of products, if specific enough to effectively brand.
The thing is with a lot of suppliers, they do tend to get a lot of inquiries, and not all of them are genuine. Plenty of time wasters around, and while they shouldn't, some will assume you are a time waster if you don't show a proven record of sales.
In these cases it is always a good idea to show them a plan, a basic business plan in essence. If you show them HOW you feel you can effectively market their products well, then they will see that you are genuine, and even though you don't have many sales under your belt, they will see that you have some firm ideas on how you will be moving forward, and that is something a genuine supplier will be interested in getting on board with.