I think kids (especailly little ones) have a lot less baggage around names than the adults. We know several people named "Megan" and the kids get confused, so we often say "so-and-so's mom, Megan" or "Megan who I work with" to differentiate. Kids have the same capacity to understand multiple people named "Daddy" or "Mommy" as they do multiple people named "John" or "Megan." While the names might mean a lot to the adults, they don't carry the same weight with the kids. My personal philosophy is that what my step-daughter calls me is between me and my step-daughter, and what she calls her mom is between her and her mom. I shouldn't be invested in what she calls her mom any more than her mom should be invested in what she calls me, because neither one has anything to do with the other, at least as far as my step-daughter is concerned. As with all things step-family related, the adults should try to set their own feelings and baggage aside and keep the children free from carrying those burdens.
However, the debate of calling a step-parent "mommy" or "daddy" is a fiercely debated one (though people seem, in general, more accepting of calling a step-dad "daddy" than they are of calling a step-mom "mommy," which I have to say is a confusing double-standard to me). Do you have a sense of how your ex would feel about your 2-yr-old calling someone else "daddy?" You could always choose a variation for your husband if your ex is going to be upset about someone else having a name he feels should be his. Papa, Dada, Dad, Pop, etc... Of course, you can also just stick with the nickname... like I said, kids don't give names the same weight we do, and your two-yr-old isn't going to think the relationship with his step-father is less than it is just because he doesn't call him "daddy." His step-dad's special place in his life is determined by what he does, not what he's called.
Personally, if your ex doesn't know the 2-yr-old at all, I would set up the webchat for the older two, but let the 2-yr-old wander in if he wants to. I can't really imagine you could explain the relationship to a two-year-old in a way that would make any sort of sense... he's not going to understand his relationship to your ex, regardless of what name you call him.