Yes and no. He does, but the possible radius is probably a little larger than you think. The other boss spawns have a larger staging area than the skull, and the bosses usually appear a little bit outside of that.

He appears in a random place semi-close to the skull, so it's pretty hard to trap him in. Plus he's great at destroying stuff.

You could try building platforms up in the trees and shooting him with frost arrows. Don't try to use any other arrows, it won't work. To get the frost arrows, craft frost resistance mead, drink it, head into the mountains, and mine some of the obsidian that's everywhere while shooting drakes for their freeze glands. It only takes 3-4 flintheads from a finewood bow to down the drakes.

You could also hire a mercenary to kill the boss for you.

Thank you ...

I've built walkways all around attached to the trees ... I plan to fight this battle from the air.
I have a stone base nearby and steps leading from the base to the walkway above.
I'm just hoping I can hold bonemass in my trap and win this war slowly but surely.

Btw ... I'm playing single player and not on a server.

And thank you for those tips. I've got some frost mead so I'll head off to a mountain beforehand and get those arrows.

Well you could have someone visit your world with Frostner and slap Bonemass silly with it. Then you could send them back to their home world with a couple stacks of iron or something.

And yeah, just staying above him and shooting him is viable; he has pretty much no way of getting you up there. The minions he spawns do have bows sometimes though. But keep in mind that he has a very high resistance to piercing damage which is what bows do. I suggested the frost arrows because they do bonus damage to anything that's wet and the swamp is always wet.

Definitely need the frost arrows. I made a hide out up a tree near the skull. Dropped the bones and jumped up there. However he appeared right above me though... So anything can happen. Had to jump down, lead him away and get back up there, it worked. So will recommend building the platforms in the trees.

I made an entire base very high up , with a portal and roof to stay dry. Made 240 frost arrows, only used around 100-120. Brought a draugr fang and hunstsmans bow just in case, the draugr never broke. Again poison resist mead just in case, didn't need.