LeeAnn,
I agree with the wise advice the ladies have shared. One idea that works along the lines of what Allison shared is to set the timer for 15 minutes for each part of the plans and as long as he works while the timer is on, at the end of 15 min. he's done with that box in the plans and you just put away that section until tomorrow. However, if he dawdles, then he gets the timer set for an additional 5 minutes on the end when the timer rings. During that 5 min., I would sit with him and really help him get done whatever is needed for that box of the plans, even to the point of writing for him or doing it orally, or scaling it back or whatever hustles it along so he's done. You want a "beat the timer" as a team type feel, like you are on his side as his helper to help him get done. So, when we're in that 5 min. after my son has dawdled time, I even talk quickly and act like I'm really rushing at that point, so my own little dawdler catches that feeling and energy (and learns what it looks and feels like to hustle). At the end of that time, he's done no matter how he performed.
I would scale back the requirements in each box, so they fit within a 15 min. block of time. Then, if needed just finish the rest the next day, or if enough was accomplished, then move on to the next day. The ladies are right that habit training is your most important goal right now.
I also agree that you don't want him off in his room doing homework all evening, as it makes you feel upset (and it would me too) and wears him out to the point of giving school a bad taste for him.

We do this rarely and even then my hubby oversees it, so it isn't me battling my son.
I'd agree that it's best to have your daughter's school in a room away from where your son is. It would also be great to give your son a break during that time using educational computer CD's, or a book on tape, or quiet reading time, or whatever is still educational but independent so that it won't make him behind if he doesn't finish it.
My own second son cannot focus on his work with distractions around him either, and he still dawdles if I'm not moving him along through his day. Some kiddos are just more this way than others.

But, my son is getting better, slowly steadily through the years. I feel your frustration and your exhaustion. So, keep sharing as you go, and we'll all chime in to help.

I just wanted you to know that you're not alone.
One last resort would be to place your son back with your daughter for a repeat trip through Beyond's left side and for the storytime from the right side of the guide (and then doing his 3 R's from the Bigger guide to beef up the program). I hesitate to recommend this, as I'm not sure growing another year older before heading back into Bigger will solve your son's problem (if he's just a dawdler at heart then it's a heart issue). However, sometimes one more year's worth of maturity can make a big difference. So, only you can weigh whether that is best at this point for you. Maybe he would be motivated by being with your daughter?
I must admit that I have been glad that my own dawdler has his own HOD guide so that his habits affect only him and not the other kiddos. This way we can systematically work through improving his habits as an individual, rather than having them rub off on my little ones or having my oldest one frustrated as he waits on my second son. With the system I've outlined above my second son does still take the bulk of my instruction/overseeing time, but he is quite joyful within his day and I just keep him steadily moving forward.
Blessings,
Carrie