STADD, I agree totally with Oubliette. IMHO you should not write suggesting how your son should behave, but rather write telling him how you will behave (i.e. with love).
Personally, having read years' worth of posts in this forum, I believe older JWs leave the Borg after self-awakening rather than being pulled out. The 'seed of doubt' needs to be planted, but ultimately it will be a personal journey. I believe you leaving should have planted that seed.
Perhaps if you need to write a letter you could write about your love for them, how much you miss them and how you hope in the future you will be able to see one another again. You could suggest 'they ask you' if they would ever like to chat about why you left, but doing it the other way round might push them further away.
However insulting the letters they send you, remember it is the WT talking. The fact they are writing means they are reaching out.
Good luck however you choose to play it.