Congrats! I was 20 weeks when we went to WDW and it was a great trip, but also the most exhausting trip we've ever taken. We did a whole lot less, but my feet ached by the time we got back to the room at night. I was so sick for the 1st trimester and beyond, until about 18 weeks. In fact, I almost cancelled the trip because I thought it would be a waste if I were still sick. Luckily the morning sickness went away about a week before we left for WDW. Also, it wasn't morning sickness, it was 24/7 sickness, but that's another story. I did have 2 days where I felt horrible in WDW, but other than that, just plain exhausted everynight. Since you've already got a little baby, you'll be going at a slow pace to begin with so that should help (though caring for the baby will probably make up for the slower pace).
What is of more pertinence to you, however, was a 2 week trip to Alaska we took when I was 6 weeks pregnant. My morning sickness started the day we left for the trip, so up to that point my only 'symptoms' were being slightly more tired and constantly having to pee. So I thought I could do everything my mom, husband, and BIL could do, but I was wrong. Once I resigned to the fact that I needed to rest a lot during the day, take a nap, and drink lots of fluids (water made me vomit, so I drank lots of apple juice, as well as Coke to help with the nausea) I had a great trip. The one thing that I shouldn't have done was gone to the dining room for dinner each night. The smells of all those foods made me sick every night. So that might be something to keep in mind about buffets and places like that.
Really, despite being tired and sick, they were 2 of the best trips we've taken anywhere. So if you go in thinking you can do less, will be very tired, etc., then you'll be happy when you can do more, rather than disappointed that you need to slow down. Have a great time with your little boy-it might be his last WDW trip as an only child so have lots of fun with him!
