An earflap hat like this
might work. Just make sure that you don't try to add too much of a tassel or ball at the top, because it needs to fit under his helmet.
Any gloves knit tightly with a thin yarn should work. Just make sure that they're not mittens. Does he wear fingerless padded bike gloves? If so, you'll need to plan for the warming gloves to fit under or over his bike gloves.
Lastly, I suggest you stay completely away from a scarf. At the most, a neckwarmer might do. Maybe something like this or this?
Biggest tip: whatever you choose, make sure you knit tightly
. Most knit items don't do much to block the wind, and that's likely his biggest enemy. If you're comfortable with it, maybe some double-knit or stranded work might help to help thicken up the fabric.