I'm wondering why you are ignoring the other two possible causes and focusing purely on the 1 in 3 option of STD?
It's a bit like you getting Thrush ( Candida infection) or BV ( Bacterial Vaginosis both of which are common female infections that are caused by ph inbalances mainly but can rarely be sexually transmitted as well) and your hubby worrying if you've cheated.

It's not the time or place my love. If you over accuse someone of doing something, even if they're innocent, it can end up driving them away.
There's a two in three chance that it has an entirely innocent cause. Hold on to that.

I'm glad he's getting medical attention and is on his way to healing.
Save your anxieties for another time.