Thanks to those of you who gave me advice on the soya issue.
I mentioned it to the leap study people today and they thought that it did sound like an allergy. In terms of the skin prick test, they said that they are more reliable for immediate allergic reactions than for the slightly delayed dietary reactions so she said the fact that it hadn't shown up certainly wasn't conclusive. Also, she said in her experience that that it was quite common for dairy allergic children to have a secondary soya allergy that manifested in sickness and not skin reactions. The advice was not to give him soya for now and wait until we assess his milk allergy again.
Am glad I trusted my instincts.