I understand some Dovii can be very shy and others very boisterous. Since you've only had him for 3 days, it's hard so say which one he is. I agree with fish addict that he is just getting accustomed to his new tank. I have done this before and the first week or so the Dovii is shy and reserved. If he is solo, he'll probably become owner responsive as mine did and eventually he'll learn to come out to the front and ask for food. Give him time. If he is solo, he'll might just become a glass banger...
If you're looking to draw him out a bit, you might want to try throwing in some live food in there...guppies or minnows work well. Dithers won't work in a 30 g since he'll just catch them and eat them.
Good Luck!