My son got a flat at 18 when I threw him out and has not wanted to come back and talk to his mum and dad. After 20yrs nearly it is so painful and last year when my dad died he gave me a kiss and a hug but when my husband went to see him to ask to stop this and just phone me or send me a card and forget the pat he said no wjat do I do?

You've done all you can do.
There was nothing wrong with throwing him out at 18.
I don't know if he is 20 now or 38 from the way you write, but either way, he knows you do love him and eventually he will learn that love doesn't mean he doesn't have to find his own way in the world.
He showed his love when your dad died. That means it's in there.
Don't demand, beg, whine, or look for explanations. Just send a few cards and notes and a little package of something you made on his birthday, and wait.

There's very little you can do, he most likely still remembers how lonely and scary that time was, whatever he did to make you decide to throw him out, looking back you made that decision based on the circumstances at that time, does he know that? Do you need to say sorry?

He's 38 now or there abouts, he survived on his own, without his parents for guidance, he probably feels he does not need a relationship with you, maybe your decision at the time was the best thing that ever happened to him, I wonder if he has considered that.

20 years is a long time to hold a grudge, without more details its impossible to even hazard a guess as to why he still won't talk to you.

Sometimes we need to make the first move, even if we feel we did the right thing, what's more important now, the past or the present and future.

Maybe writing a letter to express how much you miss him and sorry for the past, that you would love a relationship with him will help towards healing the relationship.