Question

B(OH)3 is acidic while Al(OH)3 is amphoteric in nature

Answer 1

In Boron 2p orbitals are involved

Whereas Al uses 3p orbitals due which the Al-O bonds are longer and less effective overlap. Where as B-O bonds are small and more effective overlap. Boron hydroxide does not donate proton but rather accepts hydroxide ion. 

It cannot accept protons like Aluminum hydroxide, so it is only acidic.