it depends.
How did he take to the other play groups he goes to? How long did it take for him to feel comfy with those groups?
All I know is, a child does not have to attend every play group... sometimes it just is not a good mix. Kids have an extra "radar" and if they are not comfortable, or do not like the kids/parents there for some reason (and they can't explain it, it's just a feeling they have), then that is fine. No problem. It is perfectly okay.
Sometimes, we have to take the cues of our child.
My daughter, since she was very young, has always had an extra sense about people and the vibes of certain groups/people. Sometimes she would just not like a certain groups or a person/parent. It's fine. I trusted her, even though she was very young. Once, when we visited a school for preschool, she got terrified and basically would NOT enter into the vice-principal's office. At the time I didn't know why. Later, I found out that this woman had TONS of complaints about her from other kids/parents and that actually, through word of mouth, I heard that she is actually quite "spiteful" and mean to kids, not loving.
So, I think in this situation, my daughter "knew" and just could not verbalize it. And she too, got very clingy and inward when I went into that office.
Its fine if your son does not "like" a play-group. So what. No biggie. Just like adults, sometimes we just don't like certain clubs or groups or cliques. It's okay in the big scheme of things.
Try going a couple more times.. .but if he still does not like it or acts "not himself"...then stop going. He seems to already have quite a varied play group routine, and that's fine.
Or maybe, just the time slot of that play-group is at a time when he is simply tired or needs a nap. My son behaves that way when he is tired.
And sometimes, kids just need downtime.
And sometimes, at this age, they simply get "separation anxiety" (which is normal), and/or they just don't feel like jumping right in and playing...sometimes a child just needs one on one time and not doing anything. This is also how they learn and how they learn to explore. And sometimes they are just over-scheduled. "See" what is best.
Good luck,
Susan