Poor Momo. I hope the antibiotics clear him right up and he can have his teefies cleaned, and he'll be good as new.
I'd trust your vet, but if the antibiotics don't clear up the infection I'd ask for them to aspirate the lymph node.
I truly don't want to scare you, but when my bullmastiff had giant swollen lymph nodes under his neck it was lymphoma (which we were able to treat successfully with chemo for two years), but there are many, many, many minor things that can cause swollen lymph nodes (just like on people).