What preparation can improve appetite and support growth in a 13-year-old child?

A 13-year-old is not given the typical "growth medication" without clear medical indications. Growth depends mainly on genetics, hormones (especially growth hormone), proper nutrition, sleep and physical activity.

If the problem is a poor appetite, it is worth evaluating the diet in the first place and ruling out deficiencies (e.g. iron, vitamin D, zinc) – they often affect appetite and development. Vitamin and mineral preparations for adolescents with the addition of vitamin D, zinc or omega-3 acids can be considered on an ad hoc basis, but they should be supportive, not the basis of therapy.

I do not recommend using strong "appetite" remedies without consulting a pediatrician. If the child is clearly behind their peers, has a low body weight, lack of appetite lasts longer or other symptoms appear (weakness, abdominal pain, chronic infections), a medical consultation and possible diagnostics are advisable.

If you need more personalized advice, you can use the teleconsultation here:
https://www.gdziepolek.pl/telekonsultacja/z-pediatra

I attach additional information that may be helpful:

https://www.gdziepolek.pl/artykuly/brak-apetytu-u-dzieci-i-osob-starszych

https://www.gdziepolek.pl/artykuly/leki-na-brak-apetytu