If you've got enough aspen in there that it took 30 minutes to find him. I can take a pretty good guess why he's burrowing.

The temps you've got your uth I'm assuming set at aren't reaching the surface. So he's burrowing to find the heat. And in order to get the correct temps on top of a lot of substrate would be so high. That if he did burrow again and touched the glass or bottom of the tub. Hed get burned.

Try less substrate. Like a half inch or so. And set your thermostat at the recommended temps. That way there's no chance or him getting burned. And he can also burrow to the bottom if he wishes.

Should save you a little time in trying to find him too.

Are you providing any kinds of hides? Granted boas aren't as shy as balls. I think having a hide especially as a baby is important. Maybe he was stressed and wanted to hide.

Sorry for the long post. But just my 2 pennies.