It's not clear what you are showing in the pictures, but your questions should be directed to the one doing the work. One reason you pay a contractor is they solve problems like this.

Will all the traffic to this new space go through your sons room? Maybe that needs to be considered too.

If you plan on finishing this mechanical attic space to be a habitable space, then lots of other criteria need to be met. Like windows, a compliant stair, and proper insulation to name a couple. The door shape nor it's location is not optimal for other than HVAC unit access, and the egress path going through a bedroom may not be allowed by your AHJ. Also, adding 1000sf to a house without permitting could be a can of worms in the future.

I think I see mending plates in the first pic and a lack of ridge board/beam in the second, which means you have trusses. From the looks of it probably attic style trusses, which could have been designed to be sufficient structurally for a habitable space, but trusses cannot be altered. So the ceiling at the entry would have to conform, maybe by turning the stair sideways to align within the first rafter bay, but again, the stair would need to be compliant with dimensional criteria.