Hiya lz1,
Agree with the other poster in relation to Kinect. Might not be value for the amount of use he gets out of it.
With regards to the 4GB version, you would be able to buy a HDD down the line if you so wished.
Agree above in the sense that if there is going to be no online play then there would be no need to go for the 250GB. If there is it might be worth getting due to the online demo downloads that your child could download for free. The average demo would take up about 1gb.
Also, just a note on the kinect, if you are getting, is the surrounding room sufficent enough for it to be used, might want to bear that into consideration.
It is also worth baring in mind that the 4GB is flash memory and not actually a hard drive, so certain games like Halo Reach will not run co-op mode as you need to install info to the hard drive (which you cannot do on 4gb Xbox). And as above, if extra levels became available online then you would not be able to hold the information on the 4gb.
Locke