My son lives just north of Cambridge and often works in Cambridge (musician, night work) I worry about him a lot. Slight, under six foot, blond.
Blackjack, taser, bear spray, knife.
If he is on the small side and not a fighter high quality bear spray, such as Counter-Assault, may be best.
You can find it online. They make different sizes and varieties, some of which are allowed even in restrictive states.