You need Alpro Growing Up Milk which comes in Soy or Oat, or you need Oatly Barista which is also well-fortified. Regular soya milks are not a substitute for cow milk. They make it by boiling soya beans in a pressure cooker and adding salt (also bad for toddlers). The organic ones aren't even fortified with vitamins.
Please don't do things like cutting out an entire massive food group without support from a knowledgable GP, HV and ideally a dietician as you are doing this very dangerously.
Your GP also needs to refer you to an allergy test because there's a raft of utter shite online making out that if your child has really normal digestive problems like loose nappies that they must have a milk allergy when that's basically the last of a long line of possible causes.
Without the vitamin D fortification, your child will get rickets and later, depression. Their bones won't develop properly because they won't be able to absorb calcium.
Without the calcium fortification their bones will be soft and break easily, they also won't be able to grow new bones so they'll be shorter than they should be.
Source: My child had a milk allergy when he was a baby and grew out of it with the milk ladder. I know it's hard to get it properly diagnosed, we were there, but you have to be so careful with nutrition for babies and toddlers!