Saw this, had a minor swoon. Pattern unavailable. But, I spotted that the sleeves aren’t fussy and raglan and having a vertical garter stitch will give it a widthways stretch.
There’s the pattern: 20 squares (the dotted lines are either sides of one square once it’s stitched.) The green bit is the head hole. You could probably add more squares for a polo neck, but not me.
The length of my arms and my circumference were pretty close, so I’m going to wing it and see.
(Original jumper available here: https://www.etsy.com/uk/listing/171994563/boyfriends-sweater?ref=related-0)