B.W.
I don't know if this will help but my daughter is 2 (3 in June) and she hasnt seen her dad for over a year. He lives in another state and has never been very good at being a "dad". Yet my daughter calls nearly ever man dad because she associates it with being a man sense she sees her friends dads' picking them up from daycare. I honestly wouldn't make a big deal out of it. Its common for children to relate the two, but if you feel that your ex's feelings might be hurt than you should consider encouraging your son to call your boyfriend by his first name, unless if hes going to stick around, what's wrong with 2 dads?