Welcome to the boards

! Here are some suggestions that have been really great for us.
Pick a resort with a little something for everyone since you will have your entire family with you. We have done the *** which was great on the pocket book

, but the rooms were so small

. I also feel it isn't as great for adults. My kids loved the pools and arcade, but there wasn't much to thrill me or my DH. We went on a trip like you are in April of 07 so it was the grandparents, 2 couples and the 5 kids ages 4-10. We stayed at the GF thanks to the generosity of my in-laws. This was the perfect choice because there is something for everyone

, and my in-laws in particular just loved it

. It was nice to be able to watch the fireworks almost every night from the GF's dock or our balcony. If you can afford it I highly recommend this resort.
Plan plenty of time at MK because that is really everyone's favorite. Make sure to catch parades, fireworks if you think the little one can make it, and a trip to CP for some face time with the Winnie the Pooh gang

.
Chef Mickey's is a must do for little ones. My kids absolutely love it!!!
Autograph's are a real big thing with my kids and a great way to get pics of your grandson with characters. Character meals are great for this since they come to your table instead of trying to hunt them down in the parks. However, there are some great places like Camp Minnie/Mickey where they have meet and greet areas.
Any time you go on the monorail check with the attendants to see if you can ride with the conductor. It is really a col experience and gives you a whole new perspective. Just keep in mind that only 4 people can be up there at a time so some of you will have to ride seperately.
Do a count down chart for him when you get closer to the trip to get him excited. We found when we did this it not only got the kids excited but it helped my daughter with her number recognition and counting in the process

. My kids had fun making it with me by decorating it with Disney themed stickers! At 2 1/2 that would easily be something he could do and would have a ball with.
As far as t-shirts I have an easy way to make one. I bought an iron on Mickey patch that I put on a plain white polo and then I took my fabric stamp pads and letter stamps and just stamped on what I wanted to say. It is easy, not very time consuming and you could easily put your "Grandma's little Mickey" on it. I made a matching one for my son and FIL. They really had fun looking like twins (they are very close as he is the only grandson). I also made a memory box for my 2 kids and my 3 nieces all decorated with Disney stuff and gave them to them the day they found out their grandparents were taking them. They are great and fit their tickets, buttons, small autograph books, boarding passes, and monorail pilot card. It is fun for them to be able to open up those boxes and see all those things.
I hope you have a wonderful trip!!! I have to say our family

trip was really great and a memory my IL's and kids will always treasure. We still talk about it more then a year later.