Well it wouldn't hurt to let them know, usually, they try to do something special. I'd also tell any place you make ADR's. Congrats to your brother, my oldest son graduates college in May, so I know how proud you are!!!
It wouldn't hurt to tell them! But if you want to really surprise him, try calling the Disney Florist (or visit their website)! They can make something really cute for him - and have it delivered to the room!
We had the "welcome basket" delivered for my DD - and customized a few of the items! She was so surprised -and loved all the stuff!
If you want to surprise him arrange and pay for something. So many people are celebrating special occasions that they can't do something special for everyone. It doesn't hurt to mention it but I wouldn't count on it. When it happens you have been sprinkled with a little pixie dust.
You can check out the Disney Florist, they have various things. http://www.disneyflorist.com/ My sister and I got our parents a little "treat" last year when we went to WDW, and it was waiting in their room when we arrived. I think it was $65. Here is a picture of what it was: