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Thanks all!
Since I last wrote, he's gone on to solids after showing a more than healthy interest in his Grandad's Sunday lunch! He's now putting on the expected weight gains and his health visitor's happy again.
To explain, my little lad wasn't putting weight on and his health visitor was concerned so she suggested formula but, unfortunately, he developed the allergic reaction. That left us with the problem of how to get him to put weight on if he was unable to take formula and we, mistakenly, believed he was too young to be weaned.
He gave us the answer himself on a visit to see Granny and Grandad where he became extremely curious in what was on Grandad's plate. Granny acted with unaccustomed speed and the little fellow polished off a small portion of quickly liquidised lamb & potatoes!!
He's now happily eating a very varied, and quite substantial, diet.
We've since tried him with a little of some dairy products and he seems to be tolerating them much better. Further research I've done suggests that it may take a baby's digestion a while to be capable of processing the complex proteins contained in milk. Thanks again for all your help.