That blue shirt is a start. I'm thinking you can find the cargo pants at your local Walmart, or you can use this: https://www.propper.com/mens/pants/propper-uniform-bdu-trouser.html
The Propper pants are good quality. I have many of this kind in a variety of colors.
The hat he is wearing looks a lot like an equestrian helmet with a helmet cover. Here are some options.
Helmet: https://www.amazon.com/Troxel-Liberty-Duratec-Helmet-Medium/dp/B00I0SCNUY
Helmet Covers: https://www.amazon.com/Equestrian-Riding-Helmet-Cover-Turquoise/dp/B004BWVEJE/
https://www.amazon.com/Equestrian-Riding-Helmet-Cover-LIGHT/dp/B004BWYG4Y/