The ex does not need to meet this guy after just a month!! I am divorced and started dating my current husband when my son was 4. (he's 11 now). Granted he was much younger when my husband and I started dating, but honestly, I think we went out for a few months (always when my son was at his dad's or grandma's) before I 'introduced' them and it was maybe 4 months when my husband called me, knowing I had my son with me that weekend, and said " I thought of something we could ALL go do together this weekend if you want to." I allowed HIM to take that next step- after really getting to know me.
Being a stepfather is a HUGE thing and anyone who thinks otherwise is kidding themselves. The new BF needs to take that at his own pace, not be pushed into some role by your sister's ex.
My ex had already gotten involved with a woman that I had known slightly through our circle of friends, but I didn't 'meet' her again until my ex told me he was moving in with her. THEN - because it would more seriously impact my son- I asked her to meet me for coffee and go over her 'house rules' because my son was going to be there every other weekend.
But your sister's ex is being unreasonable. If your sister is just dating this guy, he is not a permanent fixture in the son's life yet and that should be all that concerns the ex.
Frankly, the rosy idea of everyone hanging out and being 'friends' is crazy and will NOT happen. My husband and I and my ex and his new wife are all polite and get along in public and with my son and do not trash talk, etc- but we are NOT friends and it is just super unlikely.
We all definitely are interested in what is best for our son's welfare and happiness- but that is NOT the same as being 'friends' as adults with each other!
Sounds like the ex wants to have his cake and eat it too and be the 'good guy' but still run the show. If I was the guy your sis had only dated a month, that might be enough to scare me off if I wasn't super serious ( and after just a month, who knows how serious it even is really??)
A lot of people confuse the 'marriage' reality with the 'parenting' reality and they are two very different things! Our mediator gave us some advice I have always kept and found to be true. She said:
"Your marriage has failed, But that does not mean you have to be failures as PARENTS. You need to learn to separate your feelings from the marriage or the break-up from your realities as joint parents who want what is best for your child."
Tell your sis to tell her ex that until she knows the relationship is more permanent and will impact their son that he doesn't need to meet her new BF and that although she wants them to all get along, they are not FRIENDS- they are PARENTS of a child. Sorry- not the same thing!!