Why not spend the money on something useful that he will enjoy? Like a good set of subs, perhaps?
If you really have to have XM radio, you need a subscription in additional to the appropriate equipment. Good luck with that!
I'd recommend that you first consider ANY possible alternatives. If you have an iPhone (or similar) and a "free" Pandora account, you can get streaming music directly from the internet. If you have an AUX connection on your radio (most new vehicles now include one), you can connect most MP3 players as well as iPods. If you have an aftermarket receiver with a USB input, you can connect flash drives or even hard drives with space for literally hundreds of hours of music. Finally, there are now many FM stations that transmit HD with much better sound than what you get from satellite.
Consumer complaints about Sirius (Sirius and XM are now one and the same) top the list for rip-off companies. Some have described their customer service and billing practices as the worst they have ever encountered.
Why would you want to deal with someone like that?
These links outline some problems that other consumers have encountered.
http://www.consumeraffairs.com/home_elec…
http://www.my3cents.com/showReview.cgi?i…
http://www.roaddrivers.org/xm_radio_scam…
http://www.greenhybrid.com/discuss/f12/x…
http://www.elementownersclub.com/forums/…
http://www.pissedconsumer.com/latest-art…
http://www.complaintsboard.com/complaint…