G.,
I find that what works with my son (4, HFA) is to warn him at 10 minutes before he has to be done, 5 minutes, and then 2 minutes. It forestalls some of the meltdowns if he knows, instead of interrupting him in the middle and saying stop right now.
As for wanting you to be involved in what he is doing, HURRAY!! you know how hard social interaction is for some Aspies? I know that's no consolation in the moment, just wanted to cheer for him. How about a schedule? A lot of our kids respond very well to an established routine, especially if it is written down and posted. You can put one up for him and one up for you, and during those times, like getting out in the morning, or making dinner that are very busy for you, block those times into your schedule, and block "play by himself" into your son's schedule. After a little adjustment time, he will understand that this time is when Mommy makes dinner and I do puzzles. Or this time is for eating bvreakfast and mommy makes lunches. It will get to be routine for you to be unavailable for play during those times. Just make sure you block in lots of "Mommy plays with me" time, because it's important, and you will need to be able to point to that block of time when he asks during the times you can't. Then you can point to the schedule and say "This is time for mommy to make dinner. After we eat it is time for us to play together." THat way he knows he will still get what he wants, when it is time for it.
Best of luck!
Jess