I used a cotton poplin from L'ouiseau fabrics upon recommendation from my other brother... he said it would bring out his blue eyes- and that the kids these days liked looking like a grandmother's tablecloth. Our own grandmother thought it was feminine and expressed vague outrage that would have been more effective if she didn't express vague outrage over almost everything that is not British. Ah, grandma. I'm going to try and make her a shirt out of the remnants.
I considered this a potentially wearable muslin anyways, to test out his size. The medium fits well in the shoulders but it was too long for his taste. Luckily, I could hem it right away on my sister-in-laws machine to the length he likes, about 24", no shirt tucking for him. The above photo is pre-length adjustment.
I made the shirt on a weekend before the holidays whilst in the middle of moving houses so my sewing room was a bit of a mess; but I had no choice because I wanted to get it to him before his birthday. I am still arranging things to my liking so it will be a bit longer before I complete anything else.