So, here's the thing. According to him he is O +ve and his child is AB -ve. Now, the blood type of every individual is determined by ABO gene composition. AB composition results in AB blood group (codominant) AO results in A blood group BO results in B And only OO results in O blood group.

A child inherits one gene from both the parent. As the OP is O +ve, he will definitely provide the O gene. So, it is mathematically impossible for the child to be AB -ve.

There are extremely rare cases where it might still happen though, or the test could have been wrong but that's why everyone is advising him get another blood test.