Yeah. Fruit can be fresh or frozen. Soy milk, fruit, maybe a touch of honey for sweetness
Can he tolerate soy yoghurt or soy icecream? You could also add them.
Another idea would be to flavour his morning milk too. I can get a full 250mL of chocolate milk into my boys, but getting that much white milk in is painfully slow. If you buy 'add your own' stuff you can control the amount of sugar etc that gets added too.