Well, then, all you have to do is include a mention of an unusual Sorting Hat experience, specifically one where it said he should be in Slytherin, but (perhaps knowing the reputation even modern Slytherins might have) he wanted to be in Gryffindor instead, and told the Hat so. It's well established that the Hat will listen to and acquiesce (perhaps with a warning or a "you would have been a good ___") to requests to be in a different House than it recommends.