Brussels sprouts plants get huge. I'd replace the stake with a larger one, just try to poke it in the same hole to reduce root damage and tie the plant to it.
I use old cotton sheets, torn into strips, to tie up all sorts of things: sprouts, tomatoes, favas...at the end of the season it all just goes on the compost pile.

That's an awesome looking plant you have! I agree with staking it with a longer and thickish stick.

Thank you! I put a very large wooden stake in this weekend, and it's not leaning as much. It still seems to grow crooked.

Just curious. From noon until 5pm, where is the sun in conjunction to that monster plant? Is it pointing to the sun by any chance?

No, actually it's pointing (or tilting) away from it. It's shading my poor broccoli seedling that is to the right of it. I don't hold out much hope for any fall broccoli!

The lovely friable soil in your raised bed might be contributing to the list on that plant. Traditional advice over here is to tread down the soil when you plant them so they are firmly anchored through the winter. You can earth them up as well as staking. They will grow new roots from the stalk. Regarding your broccoli plant, the two are a bit too close together. A full grown sprout needs a couple of feet all around. You could also reduce the wind resistance by cutting out that healthy looking top and eating it as greens. They are delicious.

actually, there are several deceptive things about this picture. The brussel is "in" the raised bed, but I built the raised bed around it, after I planted it. My friend came up with the idea to put the board between it and the rest of the bed so I could fill the rest of the bed with soil and plant fall brassicas (on the right). I decided to put raised beds in mid-summer, and didn't want to disturb the brussel. There is at least 2 ft between plants, but since the brussel is leaning to the right it seems like there is less. But, this is my first year trying to grow a brussel sprout and I think I have beginners luck because I didn't do much except add manure and fertilize twice. I had no idea they grew so big!

As well as the other good advice, if you want larger sprouts I would suggest you pull off most of the lower leaves. This will concentrate all of that growth into the sprouts, that's in addition to removing the growing tip, tasty eating.