Welcome to JustAnswer! I am a licensed veterinarian and would be glad to help!
Are there other signs of illness?
Was this the only vomit episode?
Is he now eating?
From what you report - I am not certain it promotes the need for an ER visit.
I would be much more concerned if there was continued vomiting.
Typically the smell is dental itself, rather than systemic.
Unless you are noticing weight loss, increased thirst/urination, or other illness, it would be uncommon at this age for it to be metabolic or systemic
It is not decay, it is bacterial / tartar.
That would be a likely solution here UNLESS there are other illness signs.
You are very welcome.