Awe no, OP, this is awful behaviour from him. Not ok at all.
I'm divorced too and my bf and I both have kids, who also all get on really well. We've been together nearly 4 years and neither of us would ever behave this way! It's utterly ridiculous to say all kids should be treated equally - so are you expected to buy equivalent presents for his DC on birthdays etc. too? Spend equal on holidays and sort childcare together? Totally nuts.
I don't treat my partner's DC equally to mine because they're NOT MINE. Helping each other out sometimes is one thing, having some playdates to get to know each other (SOME of the time) I agree with, spending all possible blended time together and saying all should be equal is quite another. It's batshit, controlling, possessive and would completely turn me off.
Your son needs you, and you need time with him too, and time to yourself. This shouldn't ever be something you should be made to feel bad about, Mother's Day or not.
Personally, I'd end it. Relationships like ours are complicated enough without someone being an arse. If you're not willing to end it right now, I'd strip time with DC out completely and see what happens then. It really does sound like he's trying to move things way too fast.
For comparison, we have a lovely, close and easy relationship with each other's kids (not that this hasn't taken work). We spend some time together for sure and have done lots of holidays, but respect 100% we each need and want time alone with our kids - we have this much, much more than time all together. We also do separate holidays with our own DC, as well as one together. Just because he doesn't seem as awful as your ex, doesn't mean he's a keeper. I'm glad you're doing work on yourself, I know it's tough and painful - but you can see that this isn't right, so listen to your instincts.