Usually time is a problem. Depending on class he might not make much progress. Hopefully it is a good program.
Luckily I think (if i remember it correctly) most of the reading/writing are from a pool of words, sentences etc. So you can make him practice those. Also the civics test is a set of predetermined questions/answers so he can study them or worst case memorize the answers. Since it will be read out loud you can help him by reading the questions and him to answer them. That will be useful at the interview vs reading it by himself off a flashcard. Or he can read them out loud too That is how I practiced, drove my spouse crazy :))) So there are ways to have a smaller subset of things he can focus on vs the daunting task of "learn English for the interview".
To practice reading/writing you should be doing some books that are for children/young adults depending on his level. This is not an insult, but using those types of books he can learn the basic words and sentence structure that will make it easier later on. Also movies in English and with English subtitles so he can associate the heard words to the written form. This might be hard, but he should cut off the Spanish content for a couple of months. Forcing yourself to use a language that you are not comfortable with will help enormously. When I was a kid, I had to use a old school dictionary to translate lyrics, took forever, but with google translate now it is fast so if he doesn't know a word it is easy to look it up. that would also help with spelling since he would need to type it into the app.
I barely write with my hand anymore, so i did practice a bit since it felt alien to needing to write it down instead of typing. Also you can try to do dictation where he could write in a google doc and then the autocorrect would suggest the correct spelling or on paper and correct it later.
If you speak Spanish, it is time to cut it out when you are speaking with him. Sure if there are certain nuanced things you need to say sure, but 99% communication should be in English. When he will tell you that he dreamed in English you are on the right track. And when he notices that he thinks in English as default you are golden. (I still can't do math in English since i didn't practice that)
I wish I could go to South America for a couple of months to learn Spanish. (duolingo, taking Spanish course at a community collage didn't help much unfortunately) so in my opinion the important thing is the immersion.