DITHOR is indeed an excellent reading program that is literature based. The recommendation is to stay at the student grade level for the books you pick, because you are actually teaching your child to analyze and comprehend what they are reading. Your son going into 3 rd grade would be best to start off using level 3 books and level 2/3 workbook to complete.

I would say to start there and if you notice it not to be a challenge at all, than move to a higher level book to complete the 2/3 workbook.
As for Bigger, it is a grade program to use with a 3rd grader and being that he is already doing 4th grade work, he would be able to handle Bigger with no problem and gaining great understanding and preparation in this guide to move on to the skills required in the following guides. It is does lay a great foundation and I totally agree with you to start in Bigger!
You can indeed follow up with his own level LA (Carrie recommends R&S and we have found it to be an excellent program to use with our children as it starts very gentle and continues to build on the previous book, becoming very advanced quickly in the upper levels). What LA program have you used so far? Knowing that would help figure out where you dc would place in R&S.
Singapore math is another excellent recommendation of HOD and is indeed advanced, following it's own scope and sequence and it gets more and more challenging to enter into in older grades, from other math programs. If your wanting to switch to use Singapore math to challenge your child in math more, you could do that placement test on their website to see where your dc would place!
I hope this helps you and that you will figure out what works best for your ds.
