<edited previous incorrect Southwest info>
As previously stated Southwest will require 8 roundtrips (4 during double credit period) for a free ticket.
On AA, you earn 1,000 bonus miles for online booking, giving around 5200 miles per roundtrip and a free domestic ticket (25K miles) every 5 paid trips. On the 5th trip he would earn AAdvantage Gold status, where he would earn 25% bonus miles plus upgrade credits. This would give him a free domestic ticket with AA every 4 paid trips. AA also offers more legroom in coach and many more options for using the mileage earned than Southwest. AA flight schedules are likely more convenient than Southwest also.
Delta flys non-stop from BOS-SLC but will likely be more expensive than flights from PVD.
For a credit card that offers 1% cash back without any minimums or tiers to reach, try the no annual fee Chase Freedom Card which gives the cash back monthly as a credit on your next statement and also offers 3% back on gas purchases.
[This message has been edited by ddavis (edited 03-25-2003).]