The more you give him baby junk food, puffs, crisps, the harder it will be for you to get him to like real food.
HIs lunch isn't really a lunch. It is just random food from packets on a tray or undressed vegetables. Can't you prepare him a hot meal. A soup with a bit of past and parmesan , a piece of fish with a tablespoon of rice, roasted veggies, or if raw veggies, chop a tomato with olive oil , ... no wonder he loses interest.
Puff are Cheetos for babies. And crisps, well, ...everyone knows the attraction fried food combined with salt (or in the case of "baby crisps" powdered vegetables providing the saltiness) have .
Those snacks are nutrient void. They do nothing good. And they make him develop a preference for artificial ultra processed food.
How do you present the fruit? Small cubes of watermelon or melon are usually well liked. How do you prepare vegetables? Thin slices of zucchini roasted in the oven with rosemary and garlic are absolutely delicious.
Yes, all these things take time.His snack should be fruit. Cubed, sliced, ... Ditch those snacks and don't buy them anymore.