When to worry about slow weight gain / weight loss

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We've had a tough winter with DS (5). He's been poorly on and off and never really gets a break. But with each illness he loses his appetite and he didn't have much to lose in the first place!
But now I feel we've had a full month with very little appetite and he's looking skinny! On the verge of underweight.

Problem now is I can't get him to eat enough to put on any weight. It's like he's forgotten how to eat.

We've been to the GP for the various illnesses but I haven't mentioned the weight loss. He has allergies so struggle with high calories food. I try make food as dense as possible but he might only have 2 bites.

Do I just wait it out? I don't know what a GP would say other than to feed high calorie food.

Have you plotted his height and weight in his Red Book? How does it compare to his birth centiles?

Try to give him snacks etc. Smoothies are quite dense, can hide extra calories in his favourites. Get him to help prep. Stomach probably shrunk a bit so doesn't feel too hungry.

Just wondered too if he could be anaemic. I've heard that this can affect appetite and growth?