Struggling with a picky eater is really a challenge for us, parents. I suggest itanong mo sa pedia ng kid mo kung ano ang magandang appetite booster. I know several brands like Mosebon, Restor, Appebon, Propan TLC etc. You may also want to consider something that improves your kid's gut nutrition? Consider if mahina ang kanyang katawan sa pag-absorb ng mga nutrients. For that, you may try Biobita.
If you want his appetite to increase naturally, sabay sabay kayo umain ng gulay at prutas palagi. Be a good role model and insist until he learns to eat them. You may also make eating fun by incorporating variety of designs and colors.
As for milk, you may want to consider other options like soy milk, almond milk, cow's milk, etc. Ask your pedia about this as well.
I hope this helped you.
Magbasa pa