[quote=lovelee]I’m wondering, is this a mortal sin for him and is this a mortal sin for me, since I am also involved in the act?
It certainly is grave matter for your husband to do this. Whether or not it is a mortal sin depends on whether he does so with full knowledge and deliberate consent. As for you, it is not a sin at all for you because he does this against your will, and you have made clear to him that it is against your will.
[quote=lovelee]Secondly I don’t know how to “convince” him this is wrong and is this my responsibility to do so?
Withdrawal is a form of birth control. The Church’s strictures against birth control apply equally to withdrawal. Withdrawal, in fact, is a form of birth control specifically condemned by the Bible (Gen. 38:9-10). For analysis of this Scripture passage, I recommend the article Onan’s Real Sin by Fr. Brian W. Harrison.
[quote=lovelee]Thirdly, since this is a sin (not sure if it is mortal), and I know that he is still receiving Communion, should he not be?
If he has committed a mortal sin by this act – keeping in mind all three conditions (grave matter, full knowledge, deliberate consent) must be met for mortal sin to be committed – no, he should not be receiving Communion until he goes to confession. You may certainly explain this to him, but if he chooses to go despite knowing he should not, you cannot prevent him from receiving Communion. All you can do at that point is to pray for his conversion.
Vademecum for Confessors by the Pontifical Council for the Family
The Secret Diary of Elisabeth Leseur by Elisabeth Leseur