The eyes have been drawn very well and the proportions are quite good. The inner corners of his eye however could be slightly more rounded as children's eyes tend to have a much broader curve than adults, missing this curvature can age a child's face.
I have also drawn in little arrows where the highlights could be lifted out and refined further. This will give the illusion of moisture. Deepening the darker values as well will create a much stronger contrast between the darkest and lightest values, creating the illusion of depth and more 3 dimensional eyes. His left eyebrow in the reference photo is very flat along the top from the bridge of the nose to the outer edge of the pupil. In the portrait drawing, this area has crept up above the blue line on the reference, which throws the likeness off somewhat.
His neck in the drawing has crept much too far to the right, in reality it should be lined up roughly with the outer edge of his nostril. The shape of his cheek is slightly out too far (in the drawing right where his neck meets with his cheek). I drew a curved line on the reference photo as well which indicates where the shadow on his cheek begins. The artist has of course added this curvature, but by using a value closer to 12 (it is currently around a 3) and carrying it down to his chin, his face would really fill out giving him that cute pudgy face that young kids have and would really help again to create a more 3 dimensional image. For the lips, just a slight tweak on his lower left lip is needed. In the drawing, his lip curves up towards his left nostril and really tapers off quickly when in fact it should remain slightly curved downwards extending out all the way to the edge of his lip.
Again these are all just few suggestions to perhaps improve on this portrait as it is an excellent portrait already :) I am looking forward to seeing more of this artist's work!