Our expert says:
Children who have a lactose intolerance and cannot drink milk can often still eat yoghurt because most of the lactose (milk sugar) is converted by microorganisms when the yoghurt is fermented. But if your lad is allergic to both milk and soya, then he is probably not reacting to the lactose, but to a protein in the milk that also occurs in the soya. If this is the case, then he would also react negatively to yoghurt. I don't know what symptoms he develops when he has milk or soya (asthma, diarrhoea, rashes, etc), but if the reaction is not life-threatening (e.g. like asthma) then you could give him a very small portion of yoghurt (1 teaspoon) as a "challenge" and monitor how he reacts. If the symptoms don't appear then he may not be allergic to yoghurt and you can repeat the challenge a few times. If after 2-3 such introductions of yoghurt he has still not reacted, then you will probably be able to give him small quantities of yoghurt.
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