I can see why this is so hard darling. One thing I do know about your lad is he has a heart the size of Texas and has a beautiful sensitive side that not a lot of people get to see.
I think what is needed here is a talk about things before they happen. He will always remember how badly you were treated - that's no bad thing. It gives him experience of 'red flags' to look out for. Explain to him that the last thing you want is some idiot messing with your head again but, also explain to him that you will eventually want a man in your life. There is nothing wrong with telling him that you are lonely sometimes. He is also old and wise enough to know that sexual relations come into 'friendships' too :-) How long has he been with that lovely girl of his now?
I think if you explain how much you admire and respect the way he has protected you and looked after you and how proud you are of him it lets him know he will always be the special man in your life. Tell him that when the time comes and you do meet someone, you would value his opinion on the man. Reassure him that he is the first person that will get the opportunity to meet your new man. Suggest that they could actually go for a pint or 2 together to get to know each other - not as a father figure to a son but as 2 grown men who have your best interests at heart. Hey - I tell you something else too, with the size your lad is now, I bet any new man will be only too happy to shower you with chocolates and flowers :-)
But don't forget - this conversation doesn't have to be all in one go. If you feel him getting a bit tetchy or fidgety with it, just leave it at that for one day. But let him know that you will be wanting to talk to him again because he is important to you and his feelings are important. He will eventually see how important it is for you love. You'll see - just take it slow.