It will hurt his grades. Most common core math problems start with "using method X" and end with "show your work". They aren't interested in getting the right answer, they care more about your process. Which might be okay if they didn't come up with needlessly complex and ridiculous processes to solve simple problems.
Like how they want you to find the sum 23 + 28. kids are told to subtract 3, add 2, add 20 + 30 = 50, add 3, subtract 2 to get 51. Five steps, not kidding.
The funny part of this is that when I have to solve a complex arithmetic problem in my head, I do something like this example: make adjustments to some of the terms until the operation is easy, then undo the adjustments. But doing the problem pencil-and-paper, there's no need to do it that way. Use the conventional algorithms for multiplication, division, or whatever. If the kid becomes proficient in arithmetic, he'll eventually figure out how to do it in his head the "common core" way. No one had to teach me.