LOL you just described my exact situation with my "first husband", it's how I refer to him. He is deceased also and the father of my child. If who I am speaking to knows me and my situation, then I just call him by his first name. There really isn't any other term you can use, none that I can think of.
If I'm speaking to someone who doesn't know my situation, I just say "my first husband (insert his first name)" it makes it sound a little less like you've had 12 husbands like Zsa-Zsa LOL. It also conveys that you are ok with him. I'm not, not by a long shot, but I can sorta make it sound like I am.
And the 2nd husband, well I would just refer to him as your husband that has passed. He was the real one anyway. Ooops, just realized Teak posted basically the same thing I just said, that's weird. I can't stand my 2nd husband and unfortunately he's still alive.
J/K
What you've been saying to others is perfectly appropriate, I would just add his name.