I'd be going back to see your usual doctor....someone that's only ever seen your child for 10-15mins has no idea whether your DS is doing well or not. As for saying at 4mths he should be weighing 7.5-8.0kg...well that's all dependant on what he weighed when he was born.
My DS1 was 2.9kg born and only weighed 6.4kg at 4mths...he didn't hit 8.0kg till 6mths, and he's only just over 10kg now at 19mths. He eats anything and everything but is on the go all day so I'm not worried and neither is the Doctor.
DS2 on the other hand was 3.4kg born, 6.4kg at 4mths (take in mind he had setbacks with his heart condition and lost a good bit of weight in his first month of life) and now at 8mths is 8.6kg.
Every child is different, they all have different metabolisms so I think it's stupid on the doctor's part to say he's too small and suggest solids. Only YOU know if your child is ready for solids, and even though my kids were all ready by 4mths, it certainly doesn't mean every other child will be!!
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