Relax ! Chances of contracting HIV from a single, isolated act of unprotected sex are not very significant. There is a strong possibility that your ex or the partner, who he slept with, are not infected. Therefore nobody can give anything he/she does not have.
Having said that, unprotected sex is anyhow a risk (little or significant, depending on the factors) between two partners with unknown health status...
Therefore, you should stay calm, wait for 4 weeks and get yourself tested, just to rule out the possibility. Alternately, you can ask your ex to get himself tested, if
1. That contact was his the only exposure after he separated.
2. His exposure with the third partner is 4 weeks or more old.
Having said that, never ever go unprotected with anyone, outside your mutually committed monogamous relationship