If you are in mortal sin but went to mass anyways are you in less of a sinful state?

As omission of going to mass on holy days of obligation is mortal sin (with some exceptions), John would make another mortal sin by not going. This would make his situation even worse.

TL;DR: no, for the same reason that one can't be "a little bit pregnant."

I am going to answer in reverse order, since going to mass or not seems to be the active ingredient of this question.

Making the best of a difficult situation

Is going to mass beneficial to him?

Yes, going to mass is still beneficial to someone in a state of sin.

By attending mass, even if he is unable to receive communion due to the "work in progress" regarding his marriage's convalidation, he is:

• Turning toward God, rather than away¹

• Staying connected to the faith community

• Participating in receiving the Word, and joining in the prayers of the faithful

• Keeping his priorities (thou shalt keep the Sabbath holy) in accord with the Commandments

• Acting in humility by accepting God's offer of love and friendship, even while dealing with the temporary state of sin. (That work in progress will come to fruition ...)

  CCC 1 He{God} calls man to seek him, to know him, to love him with all his strength. He calls together all men, scattered and divided by sin, into the unity of his family, the Church.

On Compounding Sin

Does it make him in a less sinful condition than if he quit going until the convalidation?

The question is asked in reverse, so I'll challenge its frame. The question "Does sin compound" was answered here. The choice to refrain from going to Mass would compound the state of mortal sin in a serious way. Beyond that, choosing to stay away from the Eucharistic celebration is also a rejection of the Faith Community (all of the other people who go to his church). It shows by his actions that he chooses not to accept their company in the communal worship embodied in the Mass. It may be a small sacrifice of ego to attend mass when one cannot receive communion, and others can, but of such small sacrifices is love, agape love, built. When it comes to love, show don't tell.

John 13:34

34 A new commandment I give unto you: That you love one another: as I have loved you, that you also love one another.

¹ Choosing to not go to Mass is a specific and mortal willful turning away from God. Sin, in general, is a turning away from God.

1850 Sin is an offense against God: "Against you, you alone, have I sinned, and done that which is evil in your sight." Sin sets itself against God's love for us and turns our hearts away from it. Like the first sin, it is disobedience ...

Repentance, as it has come to us in translation from the original Greek and Hebrew, is in origin a term that connotes a turning toward God, or a turning again toward God (which implies an initial turning away from God at some point, aka sin). As we each deal with sin, be it venial or mortal, a conscious effort to turn towards God keeps our faith priorities aligned with His will. Conversely, to willfully turn away is to reject his mercy.

CCC 1864 There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit. Such hardness of heart can lead to final impenitence and eternal loss.

CCC references are citations from the Catechism of the Catholic Church.

Anecdotal point: I've a former work colleague who attends the same church that I do. He comes to mass, with his wife and children, regularly. He is still working through the convalidation process, which has some quirks based on the circumstances surrounding their marriage. He was raised Catholic. To say that the process has been frustrating for him is an understatement, but he remains faithful and hangs in there even though he can't receive. I am looking forward to celebrating with him on the day that their marriage is convalidated.