I just want him to learn that work comes before play with a few exceptions (like holidays, etc etc etc).
I do believe for my DS its best to get it done after school, after a snack. I just know that he will have zero energy at night, or early morning. He loves just having breakfast in the morning, watching a little tv, take a shower, then school.
It's important to recognize, though, that different people function better at different times and under different situations. I work very part-time (15 or so hours/week) but those 2 days I work, when I get home, the last thing I want to do is more work (which I do bring home with me, as my job is writing, so it goes everywhere with me). For some kids, once they are distracted, they are distracted for the rest of the night, and for some, they need down-time to get more stuff done. You best recognize your child's habits and ability to get stuff done, so do what works for you, but work doesn't *always* have to come before play. Sometimes the play enables one to get the work done... and get it done more efficiently and better.
For us, playing sometimes comes first, but not always. Yesterday, for example, when dd got home, we were outside for a while putting the perennials to bed for the winter. Daylight savings ends in a few days and the weather is progressively getting worse. Yesterday was glorious weather and it would seem almost a punishment to go inside and do homework when there was so much to be learned and explored outside... while getting necessary work done. You can strike a balance between work and play, and I have found that for us, flexibility is as important an element of getting work done as anything else.