Welcome. First of all the rites of the Church are not magical spells that cause sprits peace. The rites of the Church such as the Mass, offered for the repose of the souls of the departed and the other rites that usually occur are prayers and means of Grace that Jesus gave us through which we normally recieve graces.
While a formal funeral is not something that will probably not happen without the family arranging it, you can have a Mass or Masses offered for the repose of your relative’s soul. Inform the priest of the circumstnaces. Normally a priest recieves a stipend for his time and the supplies used, succh as the candles, wine and host(s) electricity in the church etc. The church does not get a break for the electricity bill. As a priest can only accept a stipend once a day for masses offered for intentions the usual stipend of $10.00 is very reasonable. Imagine yourself working for a salary of (on average)$300 a month, and only being allowed an additional $10.00 a day from stipends. Most priests will also offer Mass without accepting a penny in stipends for those who cannot afford to offer one.
Even though you are an agnostic, you can offer prayers for the repose of his soul too. It would be a very nice charitable act on your part, and as an agnostic, it would not hurt you or the person who you wish to remember.