Hi goodyma:
Food allergies is always a pain...
My 20 month-old son drinks nothing but milk and water only. He has been eating the same food as us since he is 12 months old, of course without the added seasoning on home-cooking meals.
Have you tried him on other types of food other than congee? Alphabets Pasta, Macaroni, Whole Grain Bread, Sweet Potato, Plain Rice with chopped up vegetables and meat, Tofu, Egg...
If your baby is allergic to milk, may be you can try to supplement his diet with soy-based baby formula designed for 1.5 year olds if you are worried that he is not getting enough nutrients.
How about drinking some fresh fruit juice?
Your baby will eat when he is hungry. My son loves to chew and hate mushy food like congee and mashed potato. His appetite changes all the time too because toddlers grow in spurts.