Buying new macbook pro from US. Tax?

It's just called a "sales tax" and you can find a list of it here
Googling "United States sales tax" will find you other information as well.

And as rkauf said above the county may also tailor in it's own set as well. So check the state first and then maybe go down and check if that specific county has any measures as well. Once you find a place it may just be easier to call the Apple Store in the area for a quick check. Some places also require you to show ID and then can levy a tax if you're from out of state. I know lots of people from Washington come down to buy in Oregon because for some things they can get by on the tax but other times they can't. Depends.

If he's purchasing it inside the US and in a state that has applicable tax then yes you will need to pay it as well. It varies by state but some states, such as my own (Oregon), do not have a sales tax. Others have a rather 'high' sales tax. It all depends where he is getting it from.

If you're both unsure about it you could always call the specific Apple Store he would be getting it from and asking them if they are required to charge tax for products. Should be very simple for them to answer.

It's called sales tax. It can vary by county too, states that tax have their own rate and then counties will generally have additional tax to add. In my county sales tax is 9.25%