How does his doctor feel about his weight? I understand being concerned about seeing your little one lose weight, but more and more research is showing that being a fat baby (beyond the "normal" baby chub) often sets children up for obesity and diabetes later in life. That is separate from the milk issue, of course, but unless he is becoming unhealthily skinny, I would not try to fatten him up. I understand that fat babies are culturally the norm in the Puerto Rican community, but he is getting to an age where he's active and starting to lose some of that baby chub anyway, and may well be the healthiest thing for him (I'm NOT advocating putting any babies/toddlers on a diet. However, some parents specifically try to fatten up their healthy-weight babies by overfeeding or giving them added sugars, and this is not healthy).
As far as the milk issue goes, I wouldn't worry about it. Once a day, I'd offer a small glass of milk before offering any other beverage. If he refuses, no big deal. Give him a glass of water or juice that's diluted with about half water. Eventually, his taste for it should come back.
Nutritionally speaking, the main benefits of milk are calcium and protein. I'm sure he's getting protein in his diet, and there are other ways to get calcium -- other dairy products like yogurt or cheese and vegetables like broccoli. If you give him a dinner of chicken and broccoli and a snack of yogurt in the afternoon, he'll get the same nutritional benefits as he would from the milk.