How do you properly feed a Syrian hamster?

The most important thing is to give him a balanced seed mix, not just anything. You can get specific mixes for Syrian hamsters at pet stores, but you gotta check that there's a good mix with sunflower seeds, millet, oats and some peanuts. After that you supplement with fresh veggies like carrots, cucumber, zucchini - like 2-3 times a week is good. A bit of fruit too but sparingly because it's sugary. You've had him for two weeks now so you should be starting to see his habits, it's the right time to check if everything's going well.

As for foods to avoid, there's quite a few honestly. Skip onions, garlic, avocado and anything that's too sugary or fatty. Chocolate is a hard no, dairy products too because a lot of hamsters are lactose intolerant. Spicy foods same deal, zero. Watch out for apple seeds and pits in general too, they're toxic for them.

A question for you though - are you giving him access to fresh water all the time? Because that's just as important as food. And have you noticed if he's storing a lot of seeds in his cheeks? It's normal for Syrians but it means you need to watch that he's actually eating and there's no mold forming in his stash. If you're not sure about certain specific foods, don't hesitate to test them slowly in small amounts first.