My son's bike currently has 53/39:28/13 gearing and is hard for him on climbs, I ordered a compact 50/34 crankset in oder to ease the gearing up front.

Now the question is, should I change the cassette with a different gear combination? or should I keep the current one. I have 2 spare road cassettes that could use if needed, 23:13 and 26:12.

check out the links to my previous post authored "rltrock" -- swapping out for anticipated hill trip was a great decision (23 => 27).

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. :)

39x28=37 gear inches.

34x23=40 gear inches
34x26=35 gear inches
34x28=33 gear inches

The other thing to consider is what gear he likes to use on a dead flat road. You'd like for that gear combination to fall in the middle of his cassette when he's using the big ring. That way he'll only have to shift front chainrings as he approaches a steep hill.