We use two preparation programmes: Chosen and YouCat.
Chosen is very American, but has some excellent material. A lot of the videos are available on YouTube. This is the first one in the series and you can get to others from there.
YouCat is a much shorter course more directly linked to Confirmation itself. The stuff online isn't much use on its own but if you wanted to you can get the book for around £10.
If I were you I'd look first at the (standard/ adult) Catechism section on Confirmation and maybe go through that together with him. You can get it from the Vatican website.
1302-1303 on the effects of Confirmation would be particularly useful in writing the letter:
'1302 It is evident from its celebration that the effect of the sacrament of Confirmation is the full outpouring of the Holy Spirit as once granted to the apostles on the day of Pentecost.
1303 From this fact, Confirmation brings an increase and deepening of baptismal grace:
- it roots us more deeply in the divine filiation which makes us cry, "Abba! Father!";
- it unites us more firmly to Christ;
- it increases the gifts of the Holy Spirit in us;
- it renders our bond with the Church more perfect;
- it gives us a special strength of the Holy Spirit to spread and defend the faith by word and action as true witnesses of Christ, to confess the name of Christ boldly, and never to be ashamed of the Cross.'
You could work on those with him. What the priest is looking for is probably some basic commitment to choose the faith for himself, renew the decision his parents made in baptism etc. It would be fine, IMO, to say 'I don't fully understand what Confirmation is about yet but I would like to know more about my faith and deepen my relationship with Jesus.'
Reassure your son that the letter is not binding, he's free to decide he doesn't want to be confirmed yet at any moment. I always remind pupils of that on the day of Confirmation itself as well as at the beginning of the course.