Is he bored maybe? Mine will always demand more snacks when they're bored! Or how soon after breakfast is he asking for a snack? Maybe a more filling breakfast?
Get him to make / bake snacks with you? Then there is less time to tantrum or to constantly demand snacks because he's having fun making them. And get him involved in the clean up afterwards.
The oat/date/peanut/cocoa energy balls or 'poo balls' were easy to make, healthy and tasty - we used the What Mummy Makes recipes for baby bliss balls.
Banana pancakes
Baked egg cups - use a cutter to cut out a circle of tortilla, oil them lightly and use them to line a 12 hole cupcake tin, pour in whisked eggs and top with chopped veg herbs and cheese. Bake. They freeze well too, we call them egg cakes and because they come out a cake tin they get demolished pretty quick!
Puff pastry pinwheels make good snacks and freeze well too. Toddler can do the spreading and sprinkling. Pizza pinwheels, banana and Nutella pinwheels, apple and cinnamon, spinach ham and cheese...etc etc.
Apple slices with oat cakes and spread with a little peanut butter.
Small sandwich.
Mini breadsticks dipped in cream cheese.
Cubes of cheddar and some grapes.
Or...if you aren't feeling up to getting toddler involved with making snacks and can't be ar*ed yourself either/too tired, how about leaving a pot of snacks 'for the day' in an accessible place for him and when they're gone they are gone. He can then help himself and make choices himself and should in theory ask less because they're there and he xsn help himself (a selection pot so to speak, e.g a banana, orange segments, an apple, organix cheesy crackers, yo-yo bear, mini malt loaf, breadsticks, cheese cubes or whatever from the cupboard).