My 7yo refuses to call his stepmom 'mom' and I'm getting heat from both sides

This is getting really uncomfortable. My new girlfriend moved in about 6 months ago and she's been great with my son, but he still calls her by her first name. My ex is now using this as ammunition, saying it "proves" he's not bonding with her and blaming me for not encouraging it. My girlfriend says she doesn't care, but I can tell she's hurt sometimes.

I don't want to force the kid to call her something he's not ready for. Is this something that just takes time? Anyone else deal with this?