My crew and I work in temps over 110 from now till Oct. All the things others have posted are possibilities of his heat problems. Age, salt, heart, liver, kidneys etc.etc. If he is drinking that will dehydrate you into the next day, if he's an alcoholic or just drunk every night. Coffee will also dehydrate you.
What I do that works best for me is to SUPER HYDRATE myself the night before a long hot day outside. The next day I start drinking water from the time I wake up and throughout the day. Water will taste like medicine as you force feed yourself water.
If I do this I'm fine, if I don't I'm useless in the heat.
Also drink pedialyte or a Sports drink . Never cokes, coffee, beer etc.
A trick that was taught to me by a giant Black Man when we both worked in paving roads. He always called me " Young Blood" . He taught me to run cold water on my wrists and it would bring my core temperature down. One of the best trick ever taught me. I wonder ever happened to Rufus?