You're using acrylic yarn so when washed and dried it will be about the same size as before. The main thing is you don't want enough difference in length that you'll end up with puckering. Other than that, a bit of difference shouldn't be much of an issue IMO. Remember: It's handmade and handmade doesn't equate with perfect. He's gonna love it.
I should never overestimate my basic arithmetic skills.