Any food which he likes can be substituted for milk.
It can be fruits, normal daily food, or even some fast food, but made at home.
U may use a few alternatives like buttermilk (chaach) or plain mattha
All Milk products like paneer,curd etc are ok, the Only issue is plain milk per se.
U may try to replace it with juices as well, if nothing else works
Next Steps
Be dedicated, so don’t succumb to the child’s tantrums, use alternatives but make sure they are hygienic and preferably made at home.