Hello, and welcome!!
From what you have said, I think Beyond would be a much better fit for him. It is just the perfect program for a 2nd grade boy!--Especially if reading and writing aren't his "favorite" thing to do right now! The main books used in Beyond are so intriguing even for a busy boy who has better things to do than read!
I think Bigger would overwhelm him, and then you would probably find that you are not saving any time at all by combining them in Bigger. You would spend so much extra time trying to make him "fit" into Bigger.
As far as time goes, I think you would find that using Beyond in addition to Bigger really won't lengthen your school day very much. You will be surprised at how easy it will be. Also, your day will be so much more pleasant when your children are placed in the appropriate guide for their skill level.
Beyond is fun, fun, fun! Bigger is great, too. I am probably speaking out of turn here, but I hope you decide to go with Beyond for your son because we have loved it. I don't want you to miss out!