They often seem surprised when I watch the whole procedure, and a few have been quite visibly unnerved by it. One even retreated to the other end of the bloodmobile to whisper to the older lady in charge that I was creeping her out (unaware that I was perfectly able to hear her). The response was classic:
“Honey, if it would get more people in here, I’d tape it for them.”
I was told it was because your body is reacting with shock and eating something 30 mins before going can help.
Same thing happened to me with the tattoo, when I give blood I get lightheaded and feel awful for about ten seconds (I’ve told them, now they tip me back a little first).
Don’t even ask me about the genius plot my college roommates and I came up with back my college days.
Basically, we’d go drinking after blood drives. You’d be astonished how little it takes to get you drunk after donating a meager pint of blood or so.
/Yes, we did this. Often.
//The answer is: You get drunk a lot faster with very little alcohol.
///and yes, this is a terrible way to do things. Dangerous, irresponsible. So don’t do it, kids. I stopped after I nearly did a face-plant stumbling down the steps of the student union after a donation and realized, “Hey, I just nearly ‘donated’ all my front teeth”
All at the same time? I think I’d faint too!
I’m a champion multi-tasker!
I’d figure that if the OP’s son keeps donating even when he knows it might make him faint, he does qualify as ‘tough’.
Me as well but shhhhhh. 
My sister recently took a phlebotomy course and needed volunteers for practice sticking. The amount of blood was negligible, just enough to make sure they’d actually got a good stick and hit the vein right.
We were all greatly amused to see my big tough manly 22 year old nephew swoon.
And some juice.
And if they haven’t collected enough volume to make processing it cost effective, they’ll just dispose of it. It’s not a full pint, but it’s a set number of ounces or grams.
I found out about that because I have hard veins to stick and if they don’t get a good flow the connection can close up part way through.
The first time I gave blood was at a blood drive at work. I’m pretty sure I skipped lunch that day, but hey I didn’t know any better. They used to set up in one of our conference rooms and they had these rickety lawn-chair-like lounge chairs for the donors. When I started getting light-headed and dizzy they flipped the thing back so my feet were up and my head was down. That just made it even worse for me because then I felt like I the chair was about to either flip over backwards or collapse.
I prepared better the next few times I donated, but I still had a tendency to get light-headed about halfway through the donation. The light bulb finally went on during one donation when the phlebotomist said after a few minutes “Wow, you’re almost done! You’re a real gusher!” It was then that I realized that I was way over-doing it on the squeeze toy that they give you while you’re donating. Ever since then I’ve taken a much more leisurely pace with my squeezing and have never had a problem since. I am halfway to my sixth gallon now!
So, to the OP - tell your son that it does get better! And good for him for donating.