It is a nice gesture to be buying your son a laptop, and even better that you are interested in getting him one that is worth the money spent on it.
Toshiba and HP are both decent companies, offering good customer service and bundels of software for any new/used products.
100GB harddrive, and 1GB ram is definatly a good start, no processor can perform at it's best without the right accessories backing it up.
Comments about processors:
-as unlikely as it is.. Pentium is in a lawsuit right now, over their P3 and P4 processors, and could potentially lose their righ to these products, or have to pay some large sum of money to keep them.
It is not a large deal as they are still one of the most popular and there will continue to be updates and hardware for these processors (and related) for years, even if it becomes an individual project.
(I myself have run a desktop and a laptop with a P3 with little or no complaints. The P4 is a much superior chip to the P3)
AMD is what my desktop is currently running. Do not be confused by the numbering system, AMD's ranking (eg 3000+) is to show it's rating againts Intel. This means that although the actual reading may not be 3.0GHz, it runs equivalent to an Intel 3.0GHz processor. They are reliable and much cheaper. With overclocking a little easier (not recomended in a laptop)
They are supposed to have a slower multi-tasking capability but i am yet to see it as i am currently running more than 20 processes including a movie
A Side note, be sure that the video card, or video capability is atleast 128mb or better, i believe 256mb will be the standard in a matter of months, if it isn't already. This will keep his laptop from lagging while watching movies or playing games