The only pattern maker I know of that has a boy's shirt is Period Impressions.
If you really want to have to deal with those patterns, that is an option.
I actually think you could do quite well with this pattern by making a new drafted pattern from it. Make the back yoke fit your boy, making the squares of front and back a little narrower and a little shorter, shortening the sleeves and the adjusting the size of the "bib" and collar and you'll be fine. Shirts are loose and baggy, and long enough to tuck under the butt, so if it is not a perfect size, it will not be obvious.