KTP
There is a very regular TGV Service from Lille to Paris (every 30/60 minutes depending on day of week / time of day). Katebee is right in that any TGV tickets are for specific seats on specific trains.
It's not cheap though, 168 Eur for 2 Adults and 2 Children return.
You can get it cheaper if you book far enough in advance - these are called 'Prems' and are first come first served non flexible tickets. Also note that there are TWO staions in Lille - Lille Europe and Lille Flanders. Most (but not all) TGVs to Paris leave from Lille Flanders, the Eurostars and TGVs to other parts of France from Lille Europe. The 2 stations are near each other, but make sure you are at the right one!
A good alternative to Lille (smaller more manageable town, easier to park, slightly cheaper fares (c130 Euro return for 2a2c) )would be Arras. There are fewer TGVs (1 every hour or two) but it's a smaller town, easier to park, slightly cheaper fares etc.
Lastly is it the TGV itself that your son is interested in or is Paris a particular attraction? If the former then many trains within Brittany will be TGVs at the start/end of their journeys from Paris(albeit not going at 180 mph), especially on the lines from Rennes to Brest and Quimper.
French Trains website:
www.tgv-europe.com/en/home/?rfrr=FormulaireStandard_header_Home
or try www.raileurope.co.uk (same prices but an English Agent for SNCF so English speaking help on the end of the phone. Not totally sure if they do the prems tickets - ask them!).
Also very useful general info at excellent website here: www.seat61.com
If you want any more info happy to help, just reply back!
Ant (Mand's other half)