He'll never notice - choose a shirt you like from his laundry pile and find a sewing machine.
The open neckline, loose waist and front buttons are preserved from the original design.
For greater freedom of movement the sleeves are eliminated and the arm holes scooped even deeper- before being faced with the same fabric as the dress. A new band of fabric over the buttonholes strengthens the fastening and adds decoration with it's zig-zag top stitching. And deep side pockets sensibly backed with a dark coloured fabric, providing a handy place for tennis balls, stop watches, and locker keys.
This dress will still need some extra fabric. Re-use the sleeves for facings, but also try to find scraps for pocket linings. We have more detailed instructions at Prick Your Finger, and I am determined to make this, so I'll keep you posted on the progress.