Well here's one way ... not sure it it's the cheapest but it looks reasonably reliable. Of course it also depends on the time of your son's flights.
http://www.faurevercors.fr/LR_SATOBUS_en.html
http://www.faurevercors.fr/LR_SAT_RS_en.html
http://www.faurevercors.fr/pdf/Horaires2010_en.pdf
There are also SNCF train services from Lyon St Exupéry (airport) TGV station to Grenoble taking about 2 hours or less, sometimes direct but sometimes with one change.
Please don't shoot the messenger, but I'd strongly suggest looking at Deutschebahn's website for rail times than the SNCF one, which is frustrating and incomplete and times one out far too quickly. It often only chooses to show you train times it would like you to take rather than what's really available!
http://www.lyonaeroports.com/fre/Accueil/Acces-et-parking/Quel-transport-choisir is at the Lyon Satolas airport website and you should put DE (from) Lyon St Exupéry and in the A (to) window use the drop down window to show Grenoble. It gives comparative prices.
Click on the various modes of transport symbols or names and windows will open giving details of how to use each mode of transport to get to your destination.
Hope this helps a bit.
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