Vernon Davis -- Surprises Broncos Fan in Hospital ... 'I'm Speechless!'

Awesome moment Tuesday courtesy of Vernon Davis -- when he surprised a hardcore Broncos fan at a Children's Hospital near the Super Bowl ... and the fan was so excited, he could barely talk!

It all went down at Stanford Children's Health -- where Alex -- an 18-year-old Broncos fan -- has been courageously battling cancer and heart problems since he was 5 years old.

We're told the hospital tweeted out to several Broncos players asking if anyone could stop by -- and lo and behold, the tweets got to Vernon's people.

The tight end immediately cleared his schedule and made plans to go to the hospital -- what ensued was a pretty special moment.

We spoke with a rep for the hospital who tells us the two talked about everything from the Super Bowl to what Peyton Manning is like in real life. The two also took pics together and Vernon signed a football.

In the end, we're told the experience was a "dream come true" for Alex.

Pretty cool.