oh yeah, I know this well....
Q (12.5 mos) goes through phases: one day he is Mr. Charm Your Pants Off, the next day he is Mr. Leave Me the H*ll Alone. :LOL
Like you, though, I persist in taking him out, both for his sake (and for mine!). I want him to be exposed to other babies even if he doesn't necessarily play with them. Sometimes he sits in my lap or close to me and watches the other babies play, sometimes he & I will play together then he will slowly join the other babies, sometimes as soon as I put him down he is off looking for his 'buddies' and his favorite toys.
I would just go with the flow, if I were you. Give him what he needs, whether it be arms or nursing or space. And he can still learn to interact with other children when it is a small group!
And one last thing: don't feel like you need to make excuses for him. I've heard too many mothers say "oh, he's shy" or "she's not having a good morning" or "I'm sorry he's not being social" or "usually she's so outgoing" etc. Sheesh! Not everybody is a social butterfly, nor do I think this is necessarily a quality that everyone should strive for. Let your baby be in whatever mood s/he is in, for goodness' sake! and don't make her/him feel ashamed if s/he isn't in the mood to play with others. (I don't mean YOU specifically, UK Mom!! I mean the general "YOU", people in general.