I've been through this! My kid loves cooking and it's complicated to find something she doesn't already have, so I totally get your dilemma 😅
Some ideas that work really well: a good digital kitchen thermometer (the ones that measure the temperature of food while it's cooking) is super useful and doesn't cost much, like 15-30€ depending on the brand. Another cool thing is a set of good quality but affordable knives - doesn't need to be luxury stuff, but Victorinox or Ikea knives are brutally practical and durable. If he's into bread or pastry, a digital kitchen scale is pure gold, especially if he follows international recipes. There's also stuff like spiral vegetable cutters, professional graters or those vacuum bags for marinating meat - they're cheap and way too useful 🎯
Another option is to go for something smaller but quality, like a set of dried herbs from a good brand, special oils (premium olive oil, sesame oil), or even a recipe book if he doesn't have many. There's also cool gadgets like garlic choppers (seems like a waste of money but when you have to peel a lot of garlic it's a blessing), or heat-resistant kitchen gloves. The trick is thinking about what he might be tired of doing manually or what could make his cooking weekends even more interesting. If you told me what kind of cuisine he likes more (Asian, pastry, meat, vegetarian...) I'd have more specific ideas!