His top gear will be reduced from 110 gear inches to 104 (approx).

That may be too much of a jump for him if he regularly uses his top gear now.

His old low gear was about 38, with the new crank it will be about 33, if you change to a 23 tooth it will be 40 so he will be worse off, and a 26 tooth will give him a 35 low which is not a huge difference from his old 38.

If you have a 12-27 handy that would probably be ideal. The new 105 ten speed in 12-27 is v v nice.