You're focusing on the wrong facial features, any features you choose to include will be read as extreme features due to the low res..
For example, the laugh lines are extremely subtle in the photo, yet when it's 1 pixel thick (which is as thick as the lips) and dark it suddenly takes over and looks like an old person.
And you're making the head way too thin..
Currently it looks like aerosmith wearing a wig xD
I had a go at this, mainly to see if and how it could be done, just a quick job..
I learned a few things..
Like, 3 pixel eyes in a row is a bad idea, since it looks like a creepy stare and kinda neglects how eyes are shaped.
Also cutting away the upper lip improved the look quite a bit (it's barely visible anyways), and trying to add extra pixels to the sides just made it into a creepy joker grin.
Now this is pretty much a complete makeover, you'll have to play around a lot with shading to make it look good.
edit: Could probably use an extra pixel in height for the forehead. And the darkest outline is too dark..