I really like this, Mark! I've never used an airbrush myself, but I think it is very well done for the medium. What a wonderful painting--something to cherish over the generations for sure.
I really like his face as a whole, but I especially like how you did his glasses and hair; you managed to capture great textures with them! His eyes are very well done also. My only critique would be his sweater. To me it feels a little too flat and could use a bit more variety in shadows and highlights to give it a more 3-dimensional quality. Other than that, great work! Keep it up.