Sounds like 90 days is plenty of time. I didn't read the 258 posts, so my apologies if this has been posted before.
The world record for the fastest house is 2 hours 45 min. The Building Industry Assoc of San Diego County held a contest in 1983. Four 1500SF 3bd2bath houses with landscaping were constructed with the last two build as a race between two 350 person teams.
It is 90 days after the foundation is complete, so the clock may not have really started to run yet if they interpret the foundation to be the basement walls with first floor deck on to hold it together, as instructions say.
The real time is required in finishing the quality interior. His mom needs to relocate by May, but RUSH is a bad word, when it comes to construction. Contractors usually overbook and the weather always seems to get involved.