If by girl like tone you mean a raise in pitch when excited, then the goal is more to get him to moderate excitement as expressed vocally. A couple good ways to do that are voice acting/singing lessons (ideally it needs to be something that interests him), and meditation/breath work techniques focused on keeping calm in tense situations.
Please note – prior to puberty, your kid's voice is going to hold a higher pitch. It may even remain that way forever. What you are really shooting for is a less jarring screeching tone, rather than "enforced manliness".
The tone and wording of the question, though, is a little offensive. Girls have beautiful voices, and men with more feminine or higher pitched voices have certainly gone far in life. While doing any/all of the above steps, I'd also look at why his voice upsets you so much.
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