OP, I can see that things are desperately hard for you - it is clear from your messages both that you have suffered at the hands of this man, and that you desperately want to do the best by your son. Posting on here about it shows you are not just somebody who does whatever they feel like and tells themself they are right in all matters, but instead someone who thinking carefully and reflects on important decisions and is willing to listen to others' views. I have lots of respect for this - it's quite rare!
None of us, certainly not me, can tell you what to do for the best here, or predict the future. Probably no course of action would be without negative impact, due to the nature of your situation. However you have asked for advice, so I will share my instinctive reaction.
My husband and his mom and brother were abandoned by the husband's dad when husband was 7. No more contact after that as far as my husband knows, until he himself sort out his dad a decade and a half later.
He himself would say he carries a MASSIVE emotional wound due to this, and it's contributed to some pretty dubious behaviour on his part in the past, which has hurt himself and others. (and I am NOT saying it's an excuse for the stupid things he has done - but a contributing factor quite possibly).
The way he tells it, it was the idea of his father simply completely ceasing to care what happened to him and his brother that was the real kicker. If his dad had stayed in touch even loosely, even just letters or occasional phonecalls to his mom, he would (he claims) have found it much less damaging.
So I wonder if a father who at least bother to text your mom occasionally to ask after you and say he loves you, while undeniably totally fucking shit, is a bit less devastating than a dad who seems to have forgotten your existence entirely....
(On a practical note, if this man does stay in his son's life to any extent and you decide to allow that, I'd give your son a mobile phone perhaps earlier than you otherwise might have. Then dad can message and tell him he loves him himself if and when he wants to. My co-parent and I have my daughter a mobile quite young (closely monitored at first obviously) and it made all of us a lot happier! But six is too young sadly).