The arguments proving his mettle as an artist when sleeping (2013) are the best proof against his skills, which is flimsy proof, but the best we have.
If is was doing a scam then I would be a lot more rich than I am.
Not necessarily. Scams can be done which are not profitable. In fact, Lee Hadwin is known to have sold some of his pieces for six figures (in 2011), something that many professional artists fail to do. This is a very odd statement years after selling a piece for six figures.
If I was pretending I could not draw while awake when I really could then I would have gone to art college to develop the gift.
You might have gone to art college, but you might not. There are plenty of people who can draw at your level who have never set foot in an art college. It takes years of practice, which might have been obtained awake or asleep.
The biggest thing I've had to prove was that I could not draw while awake. But how can you really prove something like that?
You can't. This is the key. You can prove you saw something happen, but you can't prove this kind of negative.
One interesting tidbit from this article published in 2011.
Experts at the Edinburgh Sleep Center can not determine what stage of sleep Hadwin is in when he draws.
Which might mean many things, like a new stage of sleep has been discovered, or he's not asleep at all (or, if you read on, that they can't tell because he didn't visit the Sleep Center).
More interesting is that his personal website references the "Edinburgh Sleep Clinic"
... has been described by the Art world and the Edinburgh Sleep Clinic as “UNIQUE”.
Which is inconsistent in the naming of the sleep center.
The actual name of the facility in Edinburg is "The Edinburgh Sleep Centre" and the first reference to Lee Hadwin in a non-art context tells a much different story: he is "to be tested" in 2014, and that Edinburg's clinic pronounced his story "UNIQUE".
So it seems that Lee has been passing off an past pronouncement of his story as a diagnosis of his condition which is consistent with a person who's lying.
Finally, as mentioned by trustpsychology.com, his profession is a nurse. With improved exposure to the medical field, he has a better understanding of poor sleeping symptoms; making a higher fidelity ruse possible.
What confuses me is the insistence that he can't even sketch. Everyone has artistic ability, it is just not developed in most. In every one of his articles he is emphatic that he can't even "do a simple sketch" when awake. I find this highly suspect, as an awful sketch is achievable by anyone that can move a pencil over paper. Inability to achieve that would be worthy of a news article in itself.
Finally, I've yet to see any research paper on a "new sleep state" emerge from Edinburgh directly. Considering the amount of press the artist managed to create, any validation of the findings would probably create a small sensation in the Science world too. I imagine that the primary reason for no such press releases has to do with the possibility that he's still not visited the facility.
It is most likely he's a hoax.