Is there a substantial enough risk to warrant medical testing for disease/infection at this point
Yes, your husband should be tested for STDs because the woman's sexual history means that her mouth could be a source of infection. Your husband's avoidance of sexual intimacy with you shows that he is aware of this danger so, for his peace of mind and (more importantly) for your own well-being, encourage him to seek medical assistance.
You are confronting your husband's lapse with thoughtfulness and compassion. Your attitude is admirable and I commend you for this. I hope everything turns out well for you.