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.
Cheating is an option. . . . Cheaters never win and winners never cheat, but smart knitters who want to retain an iota of sanity do, cheerfully. ~~Kory Stamper