When my stepdaughters were 8 and 10, the younger one decided to stop calling me by my first name and invented "Smom" - short for "stepmom". They made me a sweatshirt that said "World's #1 Smom" and it was great. Their mother had a complete fit and tortured them over the name. After a year, it wasn't worth it, and they went back to my first name, with my encouragement. Before that, they didn't call me anything at all because they were so conflicted. It wasn't worth it.
I've been married to their father for almost 25 years, and while I would have enjoyed a term of endearment, it caused more pain for them, and it really isn't the thing that defines our relationship. It also set the tone when their mother remarried after knowing the man for 3 months - there was no competition to call him by anything other than his first name. Good thing because they got divorced after just a few years, and it would have been awful if the girls had to call him Papa and then lose him.
If your son's father is going to have a fit, then don't start something that is going to come between them!! As my husband wrote for our wedding ceremony, the girls and I understand that a relationship is more about chemistry than biology. What I'm saying is, your son's relationship with your boyfriend (and there's another inadequate term, huh??) is far more important than what your son calls him. I think you should use "Chris" UNLESS your son comes up with something else that is totally unrelated to "Dad" or "Papa". At first I thought Pop or Papa was a good idea, but if your ex is going to react, it's honestly not worth it. Hopefully Chris is mature and secure enough to know that his name is not the most important thing. If your son babbles some version of Chris' name and it sticks, that's fine. If he comes up with something else through some other route, that's fine too. But I think it is wrong to let kids "decide" what to call other people - that puts them in a bind later on when they have to call neighbors or teachers by a certain name, and you've already taught them that they don't have to!