Coffee milk is the state drink of Rhode Island, but you might be able to find Autocrat Coffee Syrup in Boston. To make coffee milk, just add the syrup to milk.
It's not bad. I like it better as an egg cream, though.
There's no egg in an egg cream, it's just syrup, milk, and soda water.
I got the Junior's glass from Restoration Hardware, so it's not like a genuine souvenir or anything. The official egg cream syrup is Fox's U-Bet Syrup, which comes in chocolate, strawberry, and coffee (but it's not as authentic for coffee milk as Autocrat).
IG will be glad to know that in Boston there's a Dunkin' Donuts on every block. There's also a lot of joints offering beef sandwiches, but I've never had one.