Hi, at 15mths theyshould be having at least 500mls a day. Theres a great web site of Heinz which shows you daily food/milk intake requirements for your toddlers, the link is:
Be guided by your toddler, if he is happy with a cup of milk, there is no stress because as long as your little one is also having cheese,fish, green veg and yoghurt, everything will be fine. I think it is all about variety.