I had this and was lucky enough to have six months with DS, three over the summer hols before he started at the local nursery at school. (I say lucky enough, it was after a rather hare raising chidcare sacking episode and we've been v broke, but still!)
He was three so a year yong, but he and I had always had group issues and he clung to me like a limpit at all parties etc, no joining in, not even for the tea, not happy at classes etc. I'd had problems when he was little because I found it really hard to look after him and intereact with the adults at the same time and felt I'd fed it acidentally and it had turned into a vicious cycle.
My strategy was to start taking him to lots of new places where there was no requirement to join in. Basically it was him and me just having fun together but going somewhere new. Soft play, one c'clock clubs, different cafes, lots of new parks, anything at all I culd and go how exciting something new. He started to ease up a lot and a new place started to be an adventure. I also got quite firm with myself about making sure he knew it was fine to be shy. Not stressing at all in social situations, let him clamp himself to my leg, ignore adults tut tutting etc and just concentrate on him. Telling him it's fine to be shy, everyone's shy and let's you and me just stay here and have a look, cuddle or whatever. Tried not to make him do stuff, no joining in, no forced hugs and kisses with family, we're just going to watch etc, you sit with me here, ignore granny blahdiblah. Over the summer things improved a lot and he's just become more confident. Think we now feed each other much more and we're both more relaxed about it. I hate going to big work dos and having to plunge in with strangers etc, so fair enough he feels that way I reckon. He started school and absolutely loved it from Day 1, hwe'd been out and about so much he was secretly dying to join in and play with groups of kids.
I'm not saying this will work for you, but it really has helped so much and we're both much more confident now and I'm sure my panic was effecting him too. We also warn him in advance about things that are coming so he's prepared and surprises do seem to catch him out. If the football was coming andit was my son I'd probably start taking him to watch other children playing football in the park at the mini league, maybe on your way to the swings or something and say, wow that looks fun do you think. Then make the effort to start playing often and say you're so good, you could go andplay with boys in the park one day. Would you like to. Build up to it slowly rather than slam him into something that's very structured and another adult is in charge. Having said all fo that I have another Mum who two months into ballet is prancing about with her DD and a bunch of 3-4s as DD won't let her leave, which is another option if you fancy a spot of footie.
Apologies for rambling, haven't got time to check what I've written! Good luck.