Apples are also pretty high in fructose, which can be hard for baby tummies to handle early on. Both my kids can only handle small amounts of it even now; well-meaning people who give them juice boxes usually means me ending up with 2 girls with tummy-aches later on, and bum rashes as well. Granny Smith apples have less sugar in them and might be easier to handle if you get to the point where you're ready to try again. Just go easy, small portions - you've gotten some great advice.
When we do apples, we most often make applesauce with them. Core, peel (if I'm not too lazy LOL), chunk, cook with a BIT of extra liquid (water is fine), no need for any added sugar unless you're using Granny Smiths (the tart green ones) and not necessarily even then. Either use an immersion blender to whiz the cooked apple pieces or pour the whole shebang into a blender that can handle boiling temps and whiz till it's the consistency you like it. Add cinnamon if you must, but it's delicious just like that, the apples cooked alone.
And as PP have mentioned, that's easy enough to freeze, maybe later on mix with meat or veg or yogurt or eat on its own when he's ready, or use in cake recipes as a substitute for oil.
