"bo" is a variety of "be" which as you mentioned it changes to "bo" with harmony with the next vowel in "khor"
"khor" is the verb stem meaning "eat"
and "tet" or "det" is the direct object of the verb meaning "to ra". this is the informal way of using direct objects.
It mostly means "may" not "should". The subjunctive mood in Persian refers to actions or states which are considered only in mind or are not real.
I hope this helps you.
verbs starting with 'be' (in spoken sometimes 'bo') are subjunctive.