I agree with Bill in just about everything. In particular I'd increase the contrast. Deepening the shadow tones will emphasise the wrinkles more and will increase the intensity of the highlights. I think all that's missing here is a degree of three-dimensional shaping...... and age! You seem to have knocked 20 years off him but given him white hair 
Bringing out the wrinkles will give him back his maturity and giving him back his black hair will restore his crowning glory!
The only actual physical change I would make is to the mouth. I think you have drawn it a little too forward looking. His head is turned away yet you have drawn its full width. There is a line extending out of the right (his right) side of his mouth which is not evident in the photo. cut this line back so it no longer extends beyond the bottom lip and I think this is all it requires. Just increasing the contrasts should take care of everything else.
Oh, and his chin is looking a little wide. At his right side you have a continuous line going from the chin and up the cheek. Actually a line ascends from the chin towards the temple - the cheek is on a separate plane behind that.
Don't give up .... this is coming along very well!
MIKE
www.SibleyFineArt.com
www.Starving-Artists.net