Help to make pattern fit a smaller kid...Please

I found this great pattern for my 7 yr old grandson who’s very much into ninja’s right now. It’s for a much larger adult size teen. The XSm size is chest 42"/length 23". My grandson is chest 25"/length 20". I’ve made a swatch to gauge & I get 17 stitches wide x 23 rows = 4". The longer length won’t be a problem as my grandson is tall. But, he’s so skinny I’m not sure how to go about cutting the pattern down. The front pocket is worked as part of the front so I can’t just take it from the sides or the middle - I know I have to evenly distribute the reduction front & back. Should I equally reduce both areas? And, I hadn’t even thought about the sleeves yet - luckily he has long arms! UGH I’ve never done this before. I’m going to email the designer but in the meantime any ideas?

Armaggedon Hoodie

If there’s a 50" chest, use half the sts and rows on that size. There’s basically no shaping for the front and back, they’re squares. Then you can seam them together and pick up sts around the armholes for the sleeves and knit down, knit straight for a couple inches, then decreasing every inch or so.

It might work using sport weight yarn and a small needle, maybe a 4 or 5 and see what gauge you get with it. You may be able to follow the smallest size as far as stitch numbers go. If it’s still a little big, size it down by using 4 or 5 less stitches per ‘size’. The difference in the cast on numbers is that many between sizes.