It sounds like Bigger Hearts will be the way to go. You will have the wonderful hands-on activities in the guide for Singapore 2A/2B. Since HOD is new for your son, try out the dictation in Bigger, being sure to follow the directions in the appendix. He will have to study the passage, taking note of spelling, punctuation, and capitalization. When he is ready, you dictate to him one clause at a time without giving any hints, and he repeats what you say then writes it down. If he does not reproduce every element correctly, you have him erase what he got wrong and copy it correctly from the model, then he repeats the same process the next day he has dictation assigned, for however many days it takes until all elements are reproduced correctly. If he has not done studied dictation before, it may be best to start him off simple. If it turns out the dictation in Bigger is way too easy, you can try dictating sentences from books he is reading instead, or get the Preparing guide and use the next level of dictation.
Bigger Hearts' target age range is 7 to 9, so he is still in the right age group. I would suggest trying Bigger as written, and only adding in the Extensions if it becomes apparent he needs more challenge

The Extension books range in level from about 4.5 to 6.7, are to be read independently by the student, and a schedule with follow-up activities in in the appendix.
As for English, HOD recommends you do most of each lesson orally or on a whiteboard and assign one portion to be done in writing in a notebook. This eliminates "busywork" and keeps lessons short

I had a 9 year old 4th grader in Bigger a few years ago and used R & S 2, but she had not had any formal grammar previously. It sounds like your son would be good with R & S 3 since he has had grammar before. Just do a lesson a day 4 days a week and that should work out fine!
Bigger Hearts will be a great intro to HOD and CM for the both of you
