I'm so sorry for your loss, but I know that you are both strong enough to do this. In your shoes, I'd be a quaking, anxious mess.
If your DS a fast runner? If so, consider a tether or harness, just so you have that extra measure of security. Consider the safety tats, but also get him an ID bracelet and you can even stitch little tags into his clothes if you want. Take a picture every morning with your cell phone when you go out, JUST IN CASE. I highly doubt anything will happen, but I know it will make you feel better to have that picture in case you need to show it to someone. Have one parent with him at all times (I know you know this but I'm just going to cover all ground). Try to teach him to always hold your hand or to the side of the stroller when walking, no matter how distracting other things are. If the place looks too crowded for your liking, one of you can pick him up and carry him for a while. It might even soothe you personally to hold him in an anxious situation.
You are justifiably anxious and stressed given your circumstances, but honestly Disney is incredibly safe. There are undercover cops (well, I say cops) just about everywhere just in case something like that happens. If you realize he's missing and can't find him in a short amount of time, start yelling his name and asking people if they've seen him. An employee will definitely come to your rescue, and if no one else approaches then run to the closest janitor or CM. This is when you can use the picture so they know exactly who they are looking for.
I've read some stories on people 'losing' their kids, and they never stay lost long. One person's story involved the child getting quite far before a plainsclothes found the child (it took about 10 minutes total). But the twist was that the child had been taught not to go with someone they didn't know or who didn't have the CM name tag, and of course this plainsclothes had neither! The kid had to be convinced (they were older and the plainsclothes ended up having to give a lot of information. Another CM might have even gotten involved) to go with them and find the frantic mother, but it all worked out in the end. That's the absolute WORST story I've heard that was not just rumor, so I honestly think that you will be OK. After a successful trip I'm sure you'll find the next ones more enjoyable.
Best of luck.
- G