It will benefit you to let your agent know all the particulars because it could put $$'s in you pockets, keep friendships and put you in complete control of your sale. An agent and seller need to have a sucessful working partnership to get the job done successfully.
Yes, by all means disclose this to the broker and decide ahead of time what if anything you will be wanting the broker to do should your buyer decide to buy. You can then have an agreement in place as to how you will compensate the broker for this particular buyer.
I would certainly disclose to him that you have a prospective buyer so that buyer can be excluded from the proposed commission structure (on the sale side only) should that buyer purchase your home. You of course can and should pay your agent a reduced fee for (if that specific buyer purchases) so your agent can administer the contract properly.