It sounds like Caudal Luring, they wiggle their tail around making it look like a worm or something to attract potential prey. I don't believe I read anything about Dums doing it, nor have I seen it in mine, but there are alot of snakes out their that do that and it is perfectly normal.
As long as the snake seems healthy and isn't exhibiting any other strange behaviours like star gazing, he's probably fine.
The above is just my opinion, someone else with more experience my be able to help more.
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1.0 Dumeril's Boa
0.0.1 Crested Gecko
...Not that the list will end there, got my sites set on a few others... snakes and other reptiles are highly addictive.