He read some were maybe here that 2 times your draw length plus 10 for bow length in inches. Since Covid I built a 66 inch Hickory ,68 inch Hickory, 70 inch white Oak, and 70 inch Elm in longer bows for target shooting. At my not quite 26 inch draw they feel longer than they need to be, but draw smooth and shoot good. They are in the 38 to 42 lb range. 70 inches 40 lbs 30 inch draw should make him a nice shooting bow.