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