Second opinion is good. You're not overreacting. It is good that you're taking the advice given. Keep us posted. He'll be ok, but it might be a long road.

How many injections has he had so far?? The usual round is 3 doses, but it does depend on the antibiotic being administered. He needs to finish whatever course has been started. Stopping then starting up again with either the same or a different antibiotic causes the bacteria to become resistant. Your vet should know this. I am slightly appalled that he even made an allusion to the final injections being not necessary. So, yes, a second opinion is good.....