Here are the requirements to be a godparent from the Code of Canon Law:
§1 To be admitted to undertake the office of sponsor, a person must:
1° be appointed by the candidate for baptism, or by the parents or whoever stands in their place, or failing these, by the parish priest or the minister; to be appointed the person must be suitable for this role and have the intention of fulfilling it;
2° be not less than sixteen years of age, unless a different age has been stipulated by the diocesan bishop, or unless the parish priest or the minister considers that there is a just reason for an exception to be made;
3° be a Catholic who has been confirmed and has received the blessed Eucharist, and who lives a life of faith which befits the role to be undertaken;
4° not labour under a canonical penalty, whether imposed or declared;
5° not be either the father or the mother of the person to be baptised (canon 874; emphasis added).
Presuming you meet the qualifications to be a godparent, your friend does not have to seek out another godparent to take your brother’s place because one godparent is sufficient (cf. canon 873). But because your brother is not confirmed and is not a practicing Catholic, he is excluded by canon law from serving as a godparent.
As for explaining to your brother, you might draw a parallel to secular mentoring roles. Just because a person might be interested in serving as a Big Brother or as a Big Sister to a disadvantaged child does not mean that the person will be allowed to serve in such a capacity unless he meets the qualifications set by Big Brothers Big Sisters of America. Analogously, a person who wishes to serve as a Catholic godparent to a Catholic child must meet the qualifications set by the Catholic Church.
As a side note, RCIA (the Rite of Christian Initiation for Adults) is for catechumens who are interested in entering the Church. Your brother is already a Catholic who is seeking to complete the sacraments. Technically speaking, he does not need to be routed through the RCIA program but he does need adult confirmation preparation. He can check with his pastor and/or his diocese to see if there are alternatives to the RCIA program for adult confirmandi.