L.P.
You could try flavoring the milk. If you are concerned about his weight, here are some websites that you might find helpful. Hope it helps :)
http://www.feeding-underweight-children.com/healthyhighca...
http://www.healthy-cooking.info/gain.htm
http://pubs.ext.vt.edu/348/348-271/348-271.html
http://pediatrics.about.com/od/nutrition/a/0806_high_calr...