A relative of mine did this-- while they were doing a similar renovation, they allowed the daughter who had the basement room to sleep on the family room couch when she wanted to, while still keeping "her space" downstairs until her upstairs room was finished. This let her wake up to sunlight but still have private space. It would be inconvenient permanently, but it would work as a stopgap.

From my limited karate experience I recall that there are ways to block and respond to attacks without causing much pain to the attacker; maybe your son should practice specifically with that in mind.