No - You can't daisy chain them - it could work but it would be a bigger headache to make it work ... do this instead ... Run the 75ft of Cat5 Cable to a small hub - you can buy a cheap 4 port 10base hub for $10-20 of ehMac or eBay - and then hookup the Xbox and computer to it.
Remember that if your son's computer is just being connected to the internet a 10Base Hub is all you need (the Xbox will never need anything higher either). If you are going to file share you might want to consider a 10/100 Hub or switch - but those are more money of course. Also to note if you buy a 4 port hub that one port is most likely used for the uplink port so you can only attach a maximum of 3 items.
the crap is piled high ... now trying to keep up