21 months old loosing weight and not growing!!!!!!!help!

Yanika • Mom of the best boy in the universe:)
Please help!!!!!!!
My baby boy was born march 2015., birth weight was 3.260kg not much but nod bad also., i was breastfeeding him till 11 month (at 12 months we found out i am pregnant again and he refused breastmilk), he started his solid at about 6,5 months... Appetite was good and weight gain was pretty stable.
12 months
Weight 10.150 
Height 79cm
Head 44.6cm
15 months
Weight 11.140
Height 84.5cm
Head 45.9
21 months (was on dec 11th)
Weight 9.130
Height 84.5
Head 45.9
My pediatrition says "well he's not dieing so thats fine.." But I dont think its fine at all., during 6 months - his grow has stopped and he lost weight!!!! Please help me., is there anything i can do to help him grow? He is eating not great but his menu is something like this
-breakfast (bottle of enfagrow formula, yogurt with oatmeal cookie) or cereal with milk. (He is not aleays finishing it!
- lunch (soup/pasta/rice/couscous/potatowith veggie or chiken (doesnt eat any other meat).. 
-dinner (cereal/formula/yogurts/kefir)... He is also getting purees that has veggies/yogurt/cereal in it... 
His appetite is not great so every meal is a challenge and i had to run around the dining area to offer him something because he doesnt even want to try anything... 
I am trying to offer him different food during the day., but he is not showing interest... I am in panic now... Since we also have preemie at home that was born month earlier .. Spent her time in NICU..... Also feeding-challenges... I feel so much stress... Sorry for being emotional...(😔