Allison - I'm smiling at your description of your ds here; I can almost picture him, and I have a dear nephew of mine in mind as I type this, as well as one of my sons. I do think this is more typical for boys than for girls. He sounds easily distracted, which I'd consider somewhat a personality trait that needs to be managed as well as possible, but that may just be part of his personality. We were just reading about Archimedes in CTC's history, and he definitely was distracted - all of the time - to the point where he forgot to eat sometimes, was so intent reading crossing a road that he was unaware of traffic, drew pictures figuring geometric problems in the oil on his arm in the baths, etc.

Still, he ended up being very intelligent and solving many mathematical mysteries the rest of us probably never would have been able to!

Sometimes highly distractible dc have so many thoughts going on they are actually thinking way more than the average Joe, and when all of those thoughts come together, they come up with some pretty amazing things!
So, I do not think your ds has ADD. I had several students diagnosed with ADD when I taught ps, and your ds doesn't match their descriptions. I think your ds is somewhat of a typical boy with a higher distractibility to consider. I do think it is wise to keep him near you, at least within the same vicinity of you.

Another good thing to do is to keep his work area uncluttered with very little around him to distract him - i.e. a quiet work place with no music, etc. on, probably no snacks/juice either during work time, but maybe later as a separate time for just a break, since that is what he'd be doing anyway during that time. My middle ds is somewhat this way. His snack time is completely unproductive and has become just a nice and needed break for him.

Breaks may be necessary for him throughout the day, so he has time where he doesn't have to be so focused. He also should probably not be near other dc that could distract him if possible. My middle ds can't be working at the same table as my oldest, as he is very distractible as well and gets nothing done near big brother. I have him work in the kitchen, and my oldest work at the dining room table. You may want to try interspersing work with breaks rather than doing school all in a row. My middle ds needs this and gets more done in less time if he just has some breaks off and on. Perhaps something like an hour of work time, then a 30 minute break, and alternating these would be worthwhile to try.
The timer is helpful as well. Setting it for the amount of time a box is supposed to take and then having it be a place he can see it counting down may help him stay on track. That really helped my middle ds begin to see how long things should take and try to work toward finishing by the time it rings. As far as sending him off to his room to work on his own or somewhere totally out of sight, that may just not work for quite awhile. I tried this with my oldest ds, who is not distractible, and he got nothing done either. Something about being out of sight makes them not work as hard, it seems.

One thing that helped my distractible middle ds is to do my teacher part with him in a block of time, then write on a marker board the independent things he had to finish, so he could check them off (or erase them) as he finished them. This seemed to help him stay more on track as he realized what he had to do and what came next. I do have to check on him often, but it is worth it as he finishes in a more timely fashion then. You probably will want to make a mental note of a halfway point to check on ds for his independent work. For example, if he is to do his grammar independently and you think it should take him about half an hour, check on him in 15 minutes and let him know he should be halfway done. This gives him some independence but helps you keep tabs on him somewhat so an entire hour is not gone with work not being finished. I'd encourage you to check out R & S English and do a good portion of it orally with him as this can be done efficiently and quickly, but if you've already firmly decided on the other grammar, perhaps portions of that can be done orally instead too.
I do understand this frustration as I have it with one my sons. I think they just need more monitoring, the timer, and an ebb and flow between work and free time to be successful. They probably will always take a little more of us too, but that's alright. God has a reason for designing them this way, and there is a purpose in it we may not ever fully understand! Who knows, maybe they'll become the next Archimedes, right?

Hope something here helps!
In Christ,
Julie