It is key to do the oral review questions in the R & S English Teacher's Guide each and every day. I know this because I forgot to do this with Wyatt when we started R & S English 3, and he couldn't remember the grammatical terms. When my loving sister asked me how he was doing with his oral review questions, I said, "What oral review questions?"

R & S English 2 doesn't have them, so I assumed 3 was the same. Once my ds started consistently having to answer the oral review questions (which he was NOT good at to begin with), I saw gradual improvements that steadily increased over the years.
Anyway, in R & S English 4, my dc did not always know the answers to these questions, but the consistent asking of the questions and going over the answers with them has helped immensely. My oldest ds just finished R & S English 6 and is ready to go on to high school. He rarely missed the oral review questions in R & S English 6, but that was due to all of the previous years in R & S English. I'd say a huge growth in retention happens from Levels 4 through 6. Making index cards for the oral review questions that he struggled with the most helped jog his memory. I'd let him look at the card if he couldn't answer a question, and I think 'seeing' it and studying it in this manner helped him remember it well over time.
I'd not add more grammar to R & S English, nor switch to another grammar. R & S English is full enough already and is an extremely effective grammar program not only for its thorough coverage of grammar, but also for its using grammar as a tool to positively impact students' daily writing ability via the R & S English writing-focused lessons. In hindsight, you are right, it would have been good to do R & S English 3. However, having been through all of R & S English 4, I cannot see going back to 3 nor repeating 4. So, I'd start R & S English 5 and do 1 lesson spread out over 2 days, doing the oral review questions 2 times and making cards to help if necessary. If your ds doesn't know the answers to the oral review questions, tell him the answer right away, but have him repeat it to you. Doing the same oral review questions the next day will further cement the answers for him. I'd only do half of R & S English 5 this year, and finish the other half the next year. I would then do the same with R & S English 6, doing it spread out over 2 years. He is young, and this will work well for him. Double-dipping in the oral review questions will eventually help things to 'stick' even though he didn't do the earlier R & S English levels. I'd also drop any extra grammar as this will be overkill - it is also covered somewhat in WWTB, so between R & S English and WWTB, he will have more than enough. I'd expect this to take a bit, as it will be a lot to recall, but give it 2 years, and I think the progress will be steady! HTH!
In Christ,
Julie