An Exclusive Listing means the Realtor will receive commission if the property is sold, no matter who brings the buyer (even if you find the buyer the Realtor would be paid the commission).
It sounds as if your Realtor is going to omit your property from the MLS (meaning the public and Realtors not working for his Brokerage will not be able to see that it's for sale) for 45 days and promote it within their Brokerage only. If it sells that way, you will only be liable for commission of 2.5% of the purchase price. If your Realtor cannot get the house sold in that 45 days of not being on the MLS, your Realtor will then put it on the MLS so everyone can see it's for sale and raise the commission to 4.5% of the purchase price.
It's up to you to decide if that is in your best interests. How fast do you want to get the house sold? (If it's in the MLS, it gains more exposure). How important is the savings on commission of the 1% of purchase price?
Whatever you decide to do, make sure the paperwork you sign is VERY clear in the wording and you completely understand what you are signing so you don't have any surprises when you close the deal.