Choosey,
You need to get separate memberships for each person.
You didnt specify how old your son is, but if he's under 11 then you can get him a junior membership at a reduced rate (see link below, and click on the different types of membership for more info).
If you want to be able to use both memberships for adults in future then it might be best to get two adult memberships now, that way you could use your sons card for a friend etc...
You will also need to keep a close eye on the Liverpoolfc.tv site about a month prior to the match as they will tell you the date of sale for that game, you need to be on the ball as they sell out fairly quickly.
Lastly, after registering/buying the memberships you need to "link" the cards together.
This is easily done online but they will also do it over the phone for you if you are having trouble.
They take a while to send out the membership cards so I'd buy them immediately if I was you.
I will PM you now with a very good site that will help you with more info, etc...