Agree 100%. You don't need a car to get from the Marriott to BU or Emerson. Plus it will give your son an idea of what he'll be dealing with as a student at either of these schools.
Also, cambridge isn't usally significantly cheaper than Boston. However, I just checked lastminutetravel.com, using dates 6/22-6/25 and nothing of substance came up ... the "good ones" are all "on request" so perhaps there's a convention or something going it.
If you really need a car, then you could probably save signficant money by staying outside the city and driving to a subway station and commuting to the schools that way.
I'm 99% sure that you'll pay to park your car at both the Mariott and Hyatt. In Boston ... the Boston Common Garage charges $8 for one hour $12, for 1-2 hours, $16 for 2-3 hours and $22 for 3-10 hours ... so you will be paying to park if you drive in to the city.
If you want to go other places ... there's outdoor lots for less money and there's the outrageous Government Center Garage which charges $15 for 40 minutes or less and quickly maxes out at $35/day if you stay more than 1hr 40 minutes.
Edited: 20 June 2011, 0:48