Serious Weight issue. At 14months age.

My son is now 2years 2months old he weights 12.5kgs and height 90cms. This seems to be normal at a glance but his weight has gone up to 13.5kgs and low to 10kgs in the past one year. Problem - for some unkown reason my son is a very irregular eater. The last week he didnt eat anything except breakfast and rest of the day it was just milk or liquid like juice etc. He rejceted everything. Even today he ate utappam for breakfast and rejected lunch and slept like that only. The 2weeks before this i.e. in January he ate very good food. Breakfast, lunch, dinner, snacks, juice but suddenly he breaks the routine. Rejects even his fav snacks and then i have to come up with an entirely new menu then he may start eating. Between this process he visibly becomes thin, looses weight. Ive tried many nutritionists, pediatrician but this pattern continues. And we go 10 steps ahead and 20steps back and results is we are in same place. I need genuine help. He was 11.7 kgs at 14months age.