I have my computer and my son's networked through a router connected to a Comcast cable modem. Although the connection is always on, I keep my browser closed until I need to use the internet. However, my son keeps his connection on all the time and often keeps several connections open, downloading or just leaves the pages open before going to school.
Some times my download speed is slower than dial up, and I blame him for hogging all the broadband.
Am I correct, that keeping several web pages open will steal the band width from a second networked computer?

Any advise would be helpful

Usually if the web page has information being refreshed, like flashing images, some pages refresh themselves every few minutes it may take up very little bandwidth - but should not effect the broadband service. Is there a noticeable difference when he does not have any pages online?

What you can try is to test the bandwidth, with and without his side connected on your PC.

While both pc's on internet, go to page
Speedtest.net - The Global Broadband Speed Test
You will see a yellow pyramid, click on that to get results

What is Speedtest.net?
Speedtest.net is a free broadband speed test with servers located all over the world. Compare your results with others and easily share them.

Now run this test while both connected and run it with just yours connect and compare the differences - this will give you download and uplaod speeds


Here is another link to compare with
CNET Bandwidth Meter Speed Test - Bandwidth Test