There are times that I have to do some computer work during the day, so I just put DD (10 months) on the floor with special "toys" that are just for the office time. She's excited to see those because they are different than her other ones in the play area. Sometimes I close the door and keep her in, but sometimes I'll open the door and let her think she's getting away with something as she crawls down the hall... ha.
I then chase after her and bring her back to play some more.
You might also put some of his toys away for a week so that they are new when they come out of hiding again.
Also, I bought a backpack carrier (with a frame) and use that to do some housework during the day. I make sure to walk by a mirror frequently so that I can have eye contact with my DD on a regular basis.
And of course, I'm constantly talking with her, even when I'm in the kitchen washing bottles and she's playing on the living room floor (20 feet away) by herself. Just the reassurance of my voice and an occasional smile keep her happy for a few more minutes. And she stay even longer if I punctuate the playing spells with a few minutes of 1-on-1 play time with mommy (tickling, giggling, etc.). Then I can go back to my work and she's happy to play for a while more. I find that about 12-15 minutes is all I can get away with before I have to change up something (the room, the toys, the playmate, etc.)