God I love these paintings! They are just gorgeous 
Yeah this could be very interesting to actually try out! I would say the only thing you will get with the grade and some proper texture values is the overall look an feel color and value wise. But to really push this, I would say you would need to paint your diffuse textures in his style as well and then apply some sweet custom post processing material magic 
Cheers!
EDIT: actually, if you look closely at Life is Strange, they use some very similar approaches to make a lot of their stuff look like an interactive painting (not characters though, but the environment). But on their end its only 3 things that I could observe that make this look: 1) pained diffuse textures still following PBR rules though; 2) very stylized normal maps that roughly follow the painted diffuse textures and 3) they used lightmass static lighting BUT they apply a filter to the baked shadow map that makes the baked shadows look slightly wobbly and hand painted. Those 3 things together are already making a huge difference and I am really a huuuuge fan of that look (and the game!^^)