My husband also panics when our 5 month old daughter cries in public (which happened a lot when she was under 2 months old if we were out at night since she cried for an hour everynight before she went to sleep). You might be surprised at how well your baby does in WDW. We went in May with extended family and friends, right before my daughter turned 3 months, and everyone in our group commented on how lucky we were to have such a great baby since she never cried during the trip. We went back in June (she was 3.5 months) and she never cried then either. Of course, we were the most shocked since at home she's very high-need and at that age she'd cry whenever she was put down. Well, in WDW she was in the Bjorn most of the day, which made her very happy, but she was also generally happy in her stroller, which is not always the case at home. But what also helped is that on vacation you are spending all of your time with your baby, unlike at home where you've got to cook, do laundry, vacuum, etc. So when they are in a fussy mood you're probably already holding them so it's not a big issue. People would ask my while we were there if it was hard taking such a young baby and I told them I found it easier than being home!
But it was true, she was a dream baby in WDW. My daughter only cried in one attraction, and it was FotLK in AK. We were sitting in the 'Lion' section and everytime they said to roar a little kid behind us would let out this incredibly loud, ear-piercing shriek after the rest of the section stopped yelling and everytime she did this my daughter started crying. So my husband ended up taking her outside, where he got himself an ice cream and waited till the show ended. In this case it was more because the noise bothered her, since it was so loud in that show that people didn't really notice her crying. What helped a lot is what many others have said-when she's start to get squimy or whiny, I'd nurse her and she'd typically fall asleep (not something she usually did at home). Also, she was easily over-stiimulated at that age, but she never had trouble falling asleep in the Bjorn or stroller when she needed a nap. Since your son is older and will probably be facing out, just turn him towards you if he gets fussy and he'll probably fall asleep. We're heading to
Disneyland next week and that's what I plan to do with my daughter since she'll be 6 months old and would not be too happy facing in the whole trip like she did in May and June.
Have a great trip, and don't worry, even if he cries, you'll see at least 10 more around you crying!