Don't do it.
You are hoping that by giving him sex, that he will trust in you. "But it is the only way I know so that he could trust all his feelings to me, hoping that I could change him and save him in that kind of thinking and the future conflict he my get into"
Trust me. A girl that gives her body up for sex, is the last thing that is going to change a guy or make him trust a girl.
Morally..you answer your own question "So I decided to get in a relationship again with him, knowing the grave sin of doing so."
"Knowing the grave sin of doing so."
Knowing what is right/wrong is very important; however, doing the right thing is even more important.
Once you knew he was in a relationship, with a child, you should have considered him "Off Limits". However, you slowly started walking down that wrong path, and you are finding that the further you travel, the more extreme the negative actions are required by your part.
Further....regardless of what he says, the fact remains he is Married with a child. For you to knowingly get involved, is terrible Karma for when you have a family one day. You may find yourself with husband and child, and one day wake up to find that your husband had sex with some other woman, and your whole world will come crashing down.. You are going to have to deal with Divorce Lawyers, you are going to have to worry about alimony payments for the child, and you are going to have to worry about who gets custody of the child and when. You then are going to have to pass the child back and forth between the two of you, is that fair to the child? No, but its also not fair to you that your husband broke all his vows and promises by sleeping with another girl. You will find it's less fair for you to stay with him, and so the child suffers because its the best evil.
So if you know the answer.. why did you post this question? I think it's because you lack the willpower to do so. You've waited this long, don't give up your values so easily. Love has a really strange way of working out, maybe this is Love testing you to see if you do the ethical thing.
Maybe something good can come between you and him, its possible. However, something even better might come your way if you let this married father go.
on a personal note....I'd be really sad if you went through with it. :-( but its ultimately your choice..hope my opinion was helpful to you in any way.