Welcome to this forum, Joy
it is not absolutely necessary to feed mice to your son's garter - they're merely nutritionally superior to many other foods
to can rest your concerns about crickets and roaches, as garters will not eat them anyway
I'm curious as to what kind of fish you are getting . . .
it sounds like your garter is eating a lot, but of course, I don't know how big the fish are - or the garter
generally I would feed a baby (up to about 8" or so) three times per week, a one-year-old a couple of times, or every five days, an adult once per week
with just fish, I'm not sure
please read the caresheets
http://www.thamnophis.com/caresheets...itle=Main_Page
and since it looks like you'll be feeding primarily fish, do look at this article as well
http://www.thamnophis.com/caresheets...?title=Thiamin,
don't worry about running to the petshop all the time - most garters can be persuaded to accept frozen/thawed fish pieces
you have quite a menagerie there
I am sure that with time you'll come to like your new addition more than you can now imagine
I hope this is enough to get you started
and ask anything
nice to meet you