Should I send him an email or
Should I talk to him on the phone? What could be the most efficient way ?
It depends on how confident you are that you can stay grounded when he's dysregulating. If you can see him in person or talk to him on the phone and hold your ground regardless of what he throws at you, then do that, but if not, you can still get your point across in writing, better actually because you can say what you need to say without being interrupted or getting involved in an emotional conversation. It's best to focus on what's best for you and your son and act accordingly, since guilt may creep in if you let it.