What are your thoughts on failure to thrive? ?

My 17-month-old son weighs 18.6 pounds and is on a special high fat diet. The nutritionist and doctors (I had to take him to a Children's Hospital 2 1/2 hours away) said to give him high calorie foods like er, ice cream, mayo, etc. to get him to "catch up" to a normal weight. I'm not sure I agree with this. He's had tests run on him, all normal, and a few more at the hospital which we'll find out the results on any time. If he's healthy, I would rather feed him well rather than set him up for a junk food diet for the rest of his life. It seems like he would be healthier at a smaller weight but with a healthier diet. Should I give him whole milk and er just to make him weigh the same as other kids?