Should he focus on missed prayers and not offer the regular Sunnah prayers?

What we understand from your question is that if someone missed prayers, and he has repented from not praying, and now he prays regularly and offers the Sunnah prayers, is it better for him to have the intention of making up the prayers that he missed instead of doing the regular Sunnah prayers, or not?

If the matter is like that, then the correct scholarly view is that if someone fails to offer a prayer until the time for it ends, with no excuse, then it is not prescribed for him to make it up and it will not be accepted from him, because Allah, may He be exalted, has ordained for each obligatory prayer a specific time. Allah, may He be exalted, says (interpretation of the meaning):

{Indeed, prayer has been decreed upon the believers a decree of specified times} [an-Nisa’ 4:103].

He has ordained that the Muslims should offer the prayer at that particular time, so if he fails to do that then he wants to offer that prayer after its time has ended, it will not be accepted from him because the Prophet (blessings and peace of Allah be upon him) said: “Whoever does an action that is not part of our matter, it will be rejected.” Narrated by al-Bukhari (2697) and Muslim (1718).

If a Muslim repents from failing to pray, then Allah, may He be exalted, will turn to him in mercy. Repentance means regretting his shortcomings and negligence, starting to pray regularly and resolving not to go back to missing prayers once again.

This repentance is sufficient, if Allah wills, and he does not have to make up the prayers that he missed, but the more supererogatory prayers (regular Sunnah prayers and others) he offers the better, and that is a sign of repenting properly.

At-Tirmidhi (413) narrated that Abu Hurayrah (may Allah be pleased with him) said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “The first of his deeds for which a person will be brought to account on the Day of Resurrection will be his prayer. If it is in order, then he will have succeeded and done well, and if it is lacking, then he will have failed and lost.

If anything is lacking from his obligatory prayer, the Lord, may He be glorified and exalted, will say: ‘Look and see whether My slave has any voluntary prayers to his credit.’ Then what is lacking from his obligatory prayers will be completed from that. Then all his deeds will be dealt with in like manner.” Al-Albani classed it as sahih in Sahih at-Tirmidhi.

See also the answer to question no. 21170 .

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:

The basic principle is that in the case of every act of worship for which there is a specific time, if a person fails to do it until its time ends with no excuse, then it will not be accepted from him. One example of that is failing to pray until the time for the prayer ends, then a person gets up to pray; it will not be accepted from him. The same applies to deliberately failing to fast one day of Ramadan with no excuse, then the individual wants to make it up after that. We say: it will not be accepted from him. "(Majmu‘ Fatawa wa Rasa’il Ibn ‘Uthaymin 12/28).

And Allah knows best.