Thanks Cookie, Frozzie and Bedlam for your concern and well wishes.
Cookie - I don't think I could put him on the bench and cook. He's normally quite active anyway so I don't put him on the bench anyway. I think the ground is the best level for him for a while....
Frozzie - no, he wasn't doing anything out of the ordinary or weird. It just seemed that when he spun around, his foot stayed still, causing the break. DD10 was very worried about him (as was DH and i) and couldn't stop thinking about it all the next day at school, given the fact he broke it just like that. We were all around and he was playing really nicely on his own and we were about to get the kids into bed. If only we had of put him to bed five minutes earlier.... The doctor said it was quite common and is called a 'toddler fracture', occurring in kids between about 18months and 3 years. Normally they do it by climbing off chairs, falling over a toy or involving stairs.
Bedlam - play dough is definitely on the agenda. He's starting to get frustrated not being about to move much (although is beginning to bottom shuffle). It's school holidays here now, so at least the other two kids can help keep him entertained.