We use any supermarket own brand soya milk - ds has been on it for 12 years and it's not gone up in price in all that time - usually 59p a carton. He's always had unsweetened but they do sweetened versions too. Can you try those?
We don't use soya milk anymore (son now intolerant to soya) but is it worth trying a tin of formula instead? Wysoy used to be about 10 a tin, and lasted us about 10 days or so iirc, I'd just make up a jug full and shove it in the fridge. It's fairly sweet but not overly, and it's got all the vitamins and everything in.
A lot of the supermarkets claim to have it but Waitrose, Ocado, sainsburies and Asda have all let me buy it and then when the delivery turns up its not available and been substituted with the regular stuff. I'll try tesco.
We can't use any nut based one cos we have a nut allergy and the rice milk isn't suitable for under fives. He hated oat milk. Anyway I'll try coconut and I've apple juice in the fridge, that sounds like a really good idea.
We've still got some formula left over (he's 19mo), so if the worst come to the worst, thanks for the suggestion.
Where are you? DS only has this on cereal now and we have 20 of these at home so happy to pass a couple your way if that would be helpful (we bulk buy from Ocado every couple of months as our local Tesco is always out of stock).