My 21 month old son throws tantrums when I talk to other moms and children- extreme social anxiety?

My son is okay if we are at the park, pool, play gym and it’s open free play and not too many other kids.

Recently tho if I meet with another mom and I’m talking with the mom and child, my son avoids being around them at all costs. Throws a huge tantrum if we go near them. For example, we all put our stuff down on a picnic table and he won’t go near it to even have his favorite snack if they are there. He also gets really overwhelmed son bigger groups and toddler groups where you interact a lot with the teacher and other children. He is starting to copy other kids tho and follow them and pretend/parallel play but more one on one. He avoids groups at the playground. He doesn’t have any signs of autism (I’m trained to evaluate child with autism so know what to look for, would admit if I thought he was at risk) but this behavior, it’s almost like he is jealous of me interacting with them. He gets really overwhelmed (buries his face into me) with more direct interaction, for example with a puppet and another adult. He does completely fine with us tho and familiar family members

It’s hard with the tantrums because I feel like I can’t meet up with any other moms. He literally throws tantrums the whole time pnce they arrive.

Any suggestions? I’ve started preparing him before saying we are going to say hello and play with such and such today. I might start showing him pictures maybe and explains they are friends and you can play with them too?

Thanks a bunch!