STR8, the letter is a good thing. If he wasn't preachy in it, and spoke of family, that means he wants to keep the door open to you. I know first hand how this goes, and I will share quickly what I can tell you.
If he does get preachy on you, realize that he can't help but be your witness Dad. Ignore it, and be polite. Avoid trying to prove him wrong. It won't work, and it will just make him work harder. YOU COULD show him or tell him what seems to be the most logical evidence that he is wrong, and it will cause him to alienate you or attempt harder to prove you wrong.
Now, what I said is hard to put into practice. It's hard to let your Dad get away with thinking he's right about everything, or maybe thats just me. But do avoid at all cost arguing with him about the things you know are right. It likely won't change his mind. I have tried it for the last 5 years with my Dad, and it hasn't helped family matters at all. Dad doesn't like me, and he told the rest of my JW family about how I am misguided. You can't prove them wrong, they won't listen.
I had a lot of good times with my Dad and other family before I started the "apostate" behaviour. We went camping and other fun things. As long as you don't try to bore a hole in their protective belief doctrine, you will still enjoy good times with them.