When the face is damaged and part of the features are lost as here, you need to be very careful and proceed slowly. Once you've changed the appearance of the person it just isn't him anymore, and the customer would be far better off with the battered original, bad as it is. Sometimes, if it's particularly bad, you just need to be firm and say it isn't do-able if you can't retain the likeness. All you'll achieve, no matter how much work you've done, is to get yourself a rep as a crap restorer if the likeness is lost. You might not notice if you lose the likeness ever so slightly, and why should you, you don't know what the guy looked like, but the customer will notice instantly and their disappointment will be obvious. Better sometimes just to be honest and turn the job down.
This one is do-able with care. The biggest problem is that crack running down the right side of his face. What you might think is a shadow on the right of his face is actually damage from dust and airborne pollutants which have worked their way under the emulsion where it's lifted from the backing paper. The right side of his nose is actually destroyed by the crack and the only solution is to try to paint it back in or it wont look right.
Some good efforts here though.
Don't forget the name of the game is
restoration -you want to keep the aged look of the photo and do the minimum of work. I'd change the uniform only as a last resort, and as has been pointed out, he's been transferred from the Royal Artillery to a different outfit.
Also his uniform has been changed from WW1 to WW2 style. What you could do though is nick bits of the new uniform - pockets etc., and blend them into the original uniform.
If I was going to have a serious go at this I'd convert it to greyscale - do the work, and then convert it to sepia tone..like so....
Have fun! If yoiu really want to get into this stuff, the only way is keep plugging.