I actually just did a similar week long trip in June. My twins were 4 months old (edited to add: born full-term) and I have two older daughters, ages 2 and 4. It was just my DH and I on the adult end. Our family did not plan on going when the twins were so young, but my husband had 3 weeks of vacation time he had to use and so we figured why not. I grew up in Florida, worked at Disney as a young adult and have visited the parks countless times over my life. I knew all the ins and outs of the parks, only planned mornings to avoid the heat and stay on the babies schedules, did valet parking whenever possible to avoid mass transportation, paid for less packed special events and only did one sit-down character meal with all of us. It was still H-A-R-D. Did we have a good time? Yes, but it was still very difficult during a time when the parks weren't that busy. We mostly focused on character meets and minor attractions.
If you end up going I would plan on splitting off with your son solo as much as you can so he can experience more "big kid" rides and just have all the attention on him for a bit. Expect chaos and do the best you can.
Some things that helped us...
We used a double stroller (for the twins) and a single stroller that the girls rotated using as they got tired. We also
ate breakfast at the hotel and packed our lunches everyday to avoid the chaos of quick ordering with so many kiddos. My husband and I would also switch in the early evenings as to who would take the older girls solo to the parks to give them some individualized touring times. The twins only toured the parks through lunch time. We also didn't take the twins on rides. We wanted to minimize germs plus the effort it takes to get them in and out of a stroller (it was too hot to use carriers).
Just one of us would then go with the older two. My oldest isn't really into roller coasters so that made it possible to go about it that way. I'm breastfeeding, but still packed a few ready made bottles of formula in case they were both hungry at the same time (tandem breastfeeding isn't for me...kudos to those that can do it!).
Best of luck!!!!