You should offer his funeral, make dua for him, fulfill his promises and act kindly towards his family and friends:
نحن عند النبي ـ صلى الله عليه وسلم ـ إذ جاءه رجل من بني سليم فقال يا رسول الله أبقي من بر أبوى شىء أبرهما به من بعد موتهما قال نعم الصلاة عليهما والاستغفار لهما وإيفاء بعهودهما من بعد موتهما وإكرام صديقهما وصلة الرحم التي لا توصل إلا بهما
While we were with the Prophet(ﷺ), a man from the Banu Salamah came to him and said: "O messenger of Allah, is there anyway of honoring my parents that I can still do for them after they die?"
He said: "Yes offering the funeral prayer for them, praying for forgiveness for them, fulfilling their promises after their death, honoring their friends and upholding the ties of kinship which you would not have were it nor for them."
— Ibn Majah and Abu Dawud
You should clear any loans that he might have. And these include any religious obligations that he was unable to fulfill:
أن امرأة، جاءت إلى النبي صلى الله عليه وسلم فقالت إن أمي نذرت أن تحج فماتت قبل أن تحج أفأحج عنها قال " نعم حجي عنها، أرأيت لو كان على أمك دين أكنت قاضيته ". قالت نعم. فقال " فاقضوا الذي له، فإن الله أحق بالوفاء
A woman came to the Prophet (ﷺ) and said, "My mother vowed to perform the Hajj but she died before performing it. Should I perform the Hajj on her behalf?"
He said, "Yes! Perform the Hajj on her behalf. See, if your mother had been in debt, would you have paid her debt?"
She said, "Yes."
He said, "So you should pay what is for Him as Allah has more right that one should fulfill one's obligations to Him. "
Further if you wish to do more you can perform voluntary financial acts of worship on his behalf as evidenced by the ahadith:
فقال يا رسول الله إن أمي توفيت وأنا غائب عنها، أينفعها شىء إن تصدقت به عنها قال نعم
[Sa`d bin 'Ubada] said "O Allah's Messenger (ﷺ)! My mother died in my absence; will it be of any benefit for her if I give Sadaqa on her behalf?" The Prophet (ﷺ) said, "Yes"
إنه لو كان مسلما فأعتقتم عنه أو تصدقتم عنه أو حججتم عنه بلغه ذلك
Had he been a Muslim and you had emancipated slaves on his behalf, or given sadaqah on his behalf, or performed the pilgrimage, that would have reached him.
— Abu Dawud
As far as you missing his funeral prayers. According to some madhabs if you missed the prayer then you can still offer it at his grave, as the funeral prayer can be offered more than once and even after burial of the deceased as evidenced by the hadith:
أن رسول الله صلى الله عليه وسلم مر بقبر قد دفن ليلا فقال " متى دفن هذا . قالوا البارحة. قال " أفلا آذنتموني . قالوا دفناه في ظلمة الليل فكرهنا أن نوقظك. فقام فصففنا خلفه. قال ابن عباس وأنا فيهم فصلى عليه.
Allah's Messenger (ﷺ) passed by a grave of a deceased who had been buried at night. He said, "When was this (deceased) buried?" The people said, "Yesterday." He said, "Why did you not inform me?" They said, "We buried him when it was dark and so we disliked to wake you up." He stood up and we lined up behind him. (Ibn Abbas said): I was one of them, and the Prophet (ﷺ) offered the funeral prayer.
Also see: https://islamqa.info/en/answers/90030/