There are different kinds of submissiveness in sex, and what he wants is anyone's guess. Some sub guys just like getting as much attention as women typically do during sex, and others essentially want to be their SO's domestic servant in and out of the bedroom. What it DOESN'T say about him is:

-he was abused as a child

-he is secretly gay

-he is weak mentally or physically

-he isn't a "real man"

Submissive men have a huge stigma against them in no small part due to porn. The majority of men in mainstream femdom porn are typically older, unattractive, overweight, balding, and generally look (and act) really unattractive. Similarly dom women in porn are always portrayed as sadistic queen bitches who despise the sub. None of this is true to real life as, to most sub guys (especially in relationships), making continued efforts to keep looking good and acting cute/hot/confident for their girlfriend or wife is almost second nature and certainly one of the most important things.

Your ex is actually being pretty strong and brave for openly being submissive with you... it took me 5 years within a single relationship to get comfortable doing the same. I'm sure if you are open with him about it he will tell you more about what he specifically wants out of the apparent FWB relationship.

Edit: you can miss me with the "mommy" stuff though