packing lists out there specifically for Disney. Here are links to a couple of them - Mousesavers and Allears.
http://www.allears.net/pl/pack.htm
http://www.mousesavers.com/walt-dis...s/time-is-money-at-walt-disney-world/#packing
That is a great age for the first visit. We took our son at 3.5 because my wife was pregnant and we wanted to get a visit in before the second one came. We took our daughter at 3. That may have been a bit too early for us but we had a beach vacation with the family so we stopped by. She had a fun time but did not remember as much as she did the next year when we had a longer stay.
We will actually be there the same arriving on the 17th and staying at POFQ. June will be hot. Plan for midday breaks that could include a swim, nap and shower. It is so much more comfortable in the evening after a shower and a change of clothes.
If you have park hoppers this is the best time to switch parks. Park hours are generally longer so you will have time to do a lot. Right now the MK is showing open until 10 PM. I expect that to be extended on most days. The last couple of trips they have done this a little before June and usually added another parade showing.
Kids have their own way of looking at things. Sometimes it's better to let them have some free time in the parks. The playgrounds may not be much different than the ones near your house but your son may want to spend time there rather do another ride. The good thing is that WDW has been improving their queues. A number of them have things that will keep them (and you) entertained while waiting.
Some attractions to consider:
Pirates League - we have not done this but it gets pretty positive reviews here.
Star Wars Training Academy - this usually fills up early. You can always just watch it. (our eldest was a big SW at a young age)
Playhouse Disney - both of kids did this when they were younger and enjoyed it.
The Boneyard in DAK is a little larger playground that will allow him to burn off some energy.
Head to Fantasyland first on an MK day. You will be able to get a lot more done then. I don't think that has changed in the last 13 years. Peter Pan fastpass early and maybe ride standby early. Some kids really like it. I keep reading the Dumbo fastpass machines are disconnected. There are two rides now which makes it move a little faster. Enchanted Tales with Belle is the new hot ticket. It is an interactive reenactment of the B&B story. My son would not have liked it at that age but my daughter would have loved it. Different personalities.
Have fun and take lots of pictures. We took video in their earlier years. There are somethings that a photo cannot capture. My daughter was into touching characters' noses. The non face ones. Chip and Dale just tweaked hers back which gave her a kick. (BTW Chip does not like to be called Dale.)