The electropositive character increases from B to Al and then decreases from Al to Tl because of

Electronegativity order : B > Tl > In > Ga > AlElectropositivity order: B < Tl < In < Ga < AlElectronegativity of 13th group elements decreases from B to Al, due to increase in atomic radii ,thereafter increases up to thallium.From Al to Ga Electronegativity is increased due to poor shielding effect by completely filled inner shell  3d10 electronic configuration of "Ga".From In to Tl the increase in electronegativity is due to effect from lanthanoid contraction .Since the electronegativity of Al is least in this group ,hence it is most electro positive element in Boron family.