16 month old with diahorrea

Help! I have a 16 month boy. 4 months ago he had a bad tummy bug, and was in hospital as he was dehydrated. Since then, about every 3 weeks he has really bad diahorrea, lasting for a week, which ends with him having really bad wind. During the week he is ill he eats very little and dinks just enough to keep him hydrated. I am currently waiting for blood tests and poo and wee tests and go back to the hospital in june but he is still not putting much weight on. He has been on soya milk since he was 4 weeks old as he screamed after every feed, and I have been keeping him on a diary free diet which I though would help, but doesn't seem too. He also was exposed to tb in the special care baby unit when he was born and was put on strong antibiotics. Whe he is well he is a very happy little boy but if this rings bells with any one I would love to hear