Rail Pass? Buy here or there?

My son is going to a Japanese language school in Fukoaka for 2 weeks. If we purchase a rail pass, are the usable on subways? And should he rent a cell phone...they are expensive..are they necessary? Please help.

If we purchase a rail pass, are the usable on subways?

No, the Japan Rail Pass is only valid on JR trains. Subways are not operated by JR.

And should he rent a cell phone...they are expensive..are they necessary?

It is completely up to you to decide whether a cell phone is necessary for you or not. Some people need them. Some don't.

I wonder if Japan Rail Pass is reccomendable for your son, since it pays off ONLY WHEN he does INTERREGIONAL travel(from Fukuoka to Osaka/Kyoto or Tokyo) and he will go to school for 2 weeks, wont he??

You should check Fukuoka Subway's discount tickets or JR's regional rail passes
http://subway.city.fukuoka.jp/eng/fare/index.html#a
http://www.japan-guide.com/e/e2361_05.html

If your son is not going to travel around, there's no point in buying a JR pass. It is valid on (almost) all trains operated by Japan Rail (including inner city trains), but not on subways. It generally only pays off if you're doing relative far displacements.
On the website of Japan Rail you can find all necessary information on the JR pass :
http://www.japanrail.com/JR_railpass.html

I do not know if it's technically necessary to rent a Japanese cell phone, that depends on the model of your own phone. Generally foreign cell phones do NOT function in Japan. You can rent a mobile phone for about 25$/week, if it makes you feel more secure (25$ won't kill you, and nobody forces you to use it as long as there is no emergency ;-). Look at :
http://www.mobilephonejp.com/

Grtz,
Filip