M.F.
First of all, you have to be VERY focused with your time while he's at school. No chatting with friends, etc. You have to pretend you're away at an office with nothing to do but work.
Second, make it a point to spend some good time with him as soon as he gets home from school. Have a snack, play a couple games, read a book. After about 30 min to an hour of "mommy time" he should be happy to do something else. Mine actually have rest time in their rooms after mommy time. I think they just need a little break after a day at preschool. After that, break out the play-doh or something that he doesn't get to play with much and let him play it next to your computer desk (put a shower curtain down to protect your carpet if there's not a table where he can play near where your computer is. Make sure you have a stash of "self-entertaining" toys. The Thomas "take-along" track, Fisher Price Little People, Geotrax. An easel and washable watercolors could be a good 20 minutes of entertainment.
Also, if you can be in the same room with him while he watches a video or TV he may be less likely to interrupt.
You might have to spend some effort training him for rest time or "room time" if he doesn't already play on his own. If this is new, start with 5 minutes (set a timer for him) and as he's successful, increase by 5 minute increments until he can play for an hour. My 2 1/2 yr old can do this, so an older child should be able to handle it. He occasionally pops out to ask me something, but he goes back in when I remind him that room time isn't over yet. It also helps if room time is followed by something fun with you. You can minimize distractions by making sure he goes potty and has a water bottle with him in his room.
Outside play is also a good way to keep mine busy. My desk is near a window, so I can send the kids out to the sandbox for some fun time. Bubbles and sidewalk chalk are also fun. My son will happily play out there by himself, so no need for hs sister to be around to keep him busy.
You'll also need training in "phone manners" -- before you get on the phone, tell him what you expect him to do, then practice it with phone calls to daddy, friends, etc. I ask mine to either be quiet or go to another room. If they need me while I'm on the phone, they are supposed to put one hand on my leg or arm and wait quietly until I can see what they need.
If you have 2 computers, teach him how to use a mouse and set him up on Starfall.com or with one of the Jumpstart CD-roms for kids. There are even JumpStarts for toddlers that don't require the ability to use a mouse. SesameStreet.com also has some games that require just pushing keys rather than using a mouse, but Jumpstart is better.
Of course, there's always the in-house sitter or "mother's helper." One time I had a deadline to meet and got a teenager to come over for a couple hours. It was WONDERFUL. She took them to the park and fed them lunch and put them down for their naps. Of course, she's unavailable now that school has started, but you might be able to find a 12-14-yr-old who would like to play with him after school. School gets out at 3:00 or 3:30, so you can still get some good work time after that. Or, if you know any home school kids that would like to earn a few extra dollars....
Hope this helps!