M.S.
I say use email and if keeping a calendar between the two of you is important, I think you could utilize Outlook Express or Yahoo?? I'm not that computer savvy but there must be something. I'd keep it simple. Either always using the phone to communicate or always using the computer. The computer is nice because you can print and save all the emails.
Also, knowing your ex wears a wire, etc.. You could use this to your advantage and for your protection as well. If you aren't doing anything wrong than I wouldn't worry about that but he does sound a bit odd.
Oh, and to echo another post. Please please do not talk bad about your husband to your son, even if your husband stoops down to that level. Your son will pay the price. I hope you find someone to help you both co-parent the way your son deserves.
Best of luck to you,
M.