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Good afternoon, I am Dr Ruff, I am a UK GP and happy to help. Thank you for contacting us today.
From what you have said, if he is more fussy with the bottle, or if there is a rash, my first thought would be cows milk allergy.
In these circumstances it is important to make sure your child is weighed regularly to ensure they are thriving. I would also consider a hydrolysed formula without the cows milk protein. You can get this on prescription from your GP which I would advise you try given what is happening.
That is my pleasure, and I think that is a sensible choice. Best of luck.