Massbay,
A previous thread on this MAFF noted that you need a certified scale, not necessarily a weigh station, to weigh a possible pin fish. The MA Fish & Game site lists requirements here:
http://www.mass.gov/eea/agencies/dfg/dfw/hunting-fishing-wildlife-watching/fishing/minimum-weight-requirements.html
Note #6. As long as you have a certified scale and a witness to the weighing, you don't need to kill any fish or drive any distance to get to a weigh station that may or may not be open. I fish some remote bodies of water, and really don't want to kill large fish if it can be helped. I had my Berkley scale certified this year by the Weights & Measures clerk in my town. The clerk stopped by my house to certify, and I made sure to make copies of the documentation and got a cert sticker for my scale. Was a breeze and only ran me $15. On the site posted above, there's a link to the Sportfishing Affidavit. Keep a couple copies in a waterproof bag when you and your son fish. That has all the info you should need to get going. Hope this helps!
-R