My son has a life threatening allergy to milk protein which meant we had to eliminate ALL milk from his diet. you can research some good alternatives on google...just type in milk free diet or something. I even found all kinds of replacements for baking so he could have cupcakes for birthday parties and soy based ice cream too.
As for the milk drinking part - my son loved the Very Vanilla Silk Soy. It was vitamin fortified with everything else he wasn't getting from a restricted diet. Especially since your child has already had the taste for milk it may be a little tricky getting him to change but in my opinion the sugar content in the soy / rice, etc milk whether it be flavored or not is far less a risk than him not getting his calcium & vitamin needs. You could certainly limit his sugar intake elsewhere.
My son liked Almond milk you can try that or coconut milk too. There are new coconut milk based yogurts that are good, and yes, fortified OJ if he'll drink it works for calcium consumption. I hope that helps. I have spent the last 6 1/2 years finding alternatives to milk and new "foods" that he can eat, enjoy and are healthy. We've had to change our whole way of cooking and living - to be sure not to cross contaminate ANY of our foods - like don't butter your toast & stick that knife in the jelly or you won't be able to give your child jelly now from that jar. Also when you cook - make sure you use olive oils not butter, don't share the spoons in your pots while cooking. We use lots and lots of tin foil to protect surfaces we are cooking on like the grill....it's a whole other life style. My son is allergic even by touch! We have to sanitize & wash our hands after everything we eat & do which has led to very good habits.
Also, tip - be sure to carry those single dose benadryl packs with you at all times in case you need them. You never know what your kid could get ahold of by accident and companies (unfortunately) MISLABEL alot of foods. Sign up for FAAN (Food Allergy Anaphylaxis Network) online and you'll get emailed alerts from them if there are recalls on foods with undeclared milk.
Watch you restaurants too - don't share the fries with him. Lots of other things are "cooked" in that fry oil - like mozzarella sticks. My son brings his food to all restaurants & they will heat it up for us. McDonalds - has milk in their fries stay away! They never told anyone they changed the recipe and a few kids had baaaaad reactions.
Sorry for the length of my reply - I just have been doing this for a long time now and have learned ALOT and when I found out my son was allergic I knew NO ONE that had any quick answers.
Hope that was helpful & good luck!