Thanks, all. I have really appreciated the thoughtful, informative, and supportive communication. These forums are a breath of fresh air compared to some of the rudeness and general insanity elsewhere.
I certainly agree with those of you who have pointed out just how shy these baby Dums can be. He's a gentle soul, and I can tell that I will need to be sensitive to him and take my time with him once I do start handling. For now he's still in his little shoebox-sized Iris tub with deep aspen substrate that he loves to burrow in. Every now and then he'll surface and hang out on his cork bark or in a bowl of damp sphagnum moss I provided - I live in a dry area, and winter humidity has been just brutal (right now ambient humidity in my house is 20%) so I can see why he sometimes likes to spend a few hours, or a few days, in his sphagnum moss.
The great news is that he is eating regularly and enthusiastically and that he made the switch to weaned rats. The first time I thawed the rat in the same bag as a quail so that it'd be scented, but he hasn't needed that for the last couple of feedings. One quirk I've noticed - and this may just be him - is that he really prefers to be fed later at night. Twice now I've offered once it's dark, at about 8pm, and he's been interested but refused. And both times when I offered again at 11pm he took the meal right away. I wouldn't normally repeat a meal offer like that, but all I can say is that I had a feeling about it. Not scientific, I know, but it worked for me so I'll take it. In each case, on feeding day he's still acting like a Boa - coming up out of the substrate and showing great interest as I approach him with his meal, so that has been wonderful.
I have some new PVC enclosures coming for other snakes in a few weeks, so will be doing some enclosure shuffling. If he is still eating regularly at that time - and I don't see why he wouldn't be - I'll move him into a larger enclosure and keep his shoebox in there as an option for him.