dating a man with a child? am I in the wrong?

I've been dating this guy who has a 17 month old son for around 7 months, maybe just under.
He has majority custody, 75% and holidays. The mother only wanted 25%, neither one of them help the other with money.
I've had very little contact with her, when I have she seemed really polite and well spoken but recently she liked one of my old pictures on Instagram, so clearly she's been looking (that's fine, I get she would probably want to know who is in her child's life).
Two days ago I posted two photos, as we went to Hawaii for a week. One photo was of my boyfriend and his son, holding hands on the beach. The second photo was of all of us on the beach together (My boyfriend was 100% happy to have that up).
Today I received a message from her on FB saying "stop acting like ur his fucking mother, just coz u put out some puss to my ex doesn't mean shit, thts my son my urs"
Has anyone had to deal with a situation like this? I'm well aware that I'm not his mother, no one can replace her, but I can't see how I can't treat him as if he was my own? I don't have children of my own, I guess I just always thought someone would be happy that their exs partner is treating their child well. I don't want to treat him like he isn't important, he is my boyfriends world and I adore them both.