Tell him that since you're now friendly enough to talk about personal shit, one of the reasons you and hubby are so well connected is because of your mutual affliction with <horrible STI X>, then thank him for being so friendly with you that it made you comfortable enough to share because most people are horrified to think of someone covered in <horrible STI symptom X> and its hard to find such an understanding friend.
After the inevitable awkward silence, wait until he is working out then offer him a drink from your bottle.
If he takes the drink, its time to find a new gym.