Mind you, this means he hasnt been tested after his “wild” phase of many casual hookups. This puts me at such a high risk.
Without giving any details, are you and your partner having unprotected sex or are you using condoms? If you are not using condoms during sex, then you already have been exposed to whatever he is carrying. Since you already tested negative after a recent testing (?) then you're fine up until that last test.
This would be concerning if you suspect he is cheating on you and possibly exposed to whatever STI he will gain during these tryst(s). Is this the unspoken concern you're having?
On the other hand, if you are using condoms, you're protected for now. But as suspected, you are concerned that if you go without (for whatever reason) you will lose that protection and this issue still stands. Plus the longer a person goes with an untreated STI, the greater the complications will be for your partner. A true lose-lose scenario for both of you.
He sounds immature. Though we don't know his age since you left that unstated. Can you drag out of him the whys he's so adamant about testing? Maybe he's stubborn and money is an issue. If that's the case, do you have access to free STI clinics? It would be ROYALLY STUPID if it's a case of pride and embarrassment and he doesn't want to know that he's already infected.
Anyways, give him an ultimatum. Get tested or get out of the relationship.