Hydrogen will be in +1 oxidation state , so Boron will be in -3 oxidation state. Hydrogen almost always remains in +1 oxidation state, the obvious exception being in metal hydrides.
6*1 = 6 .
Boron must therefore add to give -6 to balance. Since there are 2 Borons each is is in -3 oxidation state. This is expected as Boron is in group 3.