... The undershirt is definitely a thermal undershirt. Most likely long sleeve.
Doc's has a cris-cross with the belt loops in the back, similar to the angled loop in the pair listed above. I'll have to look into it a little more. It looks like he's wearing an olive military belt too, so that's interesting that Doc would be wearing what seems to be military related pants, belt and under shirt.
"Thermal", that's the word I was searching for. Thanks, Brad! It seems I've found it: it's a US Navy undershirt.
http://www.therpf.com/attachments/f24/doc-browns-2015-cargo-pants-docs_tshirt3.jpg-272424d1389380849
Roland what store did you find the Navy Thermal shirt?
Thanks
It's on ebay.com - just search for us navy undershirt. It's the first result (on the photo you can see a box with six shirts on top).